postconf - Postfix configuration parameters
postconf parameter ...
postconf -e "parameter=value" ...
The Postfix main.cf configuration file specifies parameters that control the
  operation of the Postfix mail system. Typically the file contains only a small
  subset of all parameters; parameters not specified are left at their default
  values.
The general format of the main.cf file is as follows:
  - Each logical line has the form "parameter = value". Whitespace
      around the "=" is ignored, as is whitespace at the end of a
      logical line.
- Empty lines and whitespace-only lines are ignored, as are lines whose
      first non-whitespace character is a `#'.
- A logical line starts with non-whitespace text. A line that starts with
      whitespace continues a logical line.
- A parameter value may refer to other parameters.
  - The expressions "$name" and "${name}" are recursively
      replaced with the value of the named parameter. The parameter name must
      contain only characters from the set [a-zA-Z0-9_]. An undefined parameter
      value is replaced with the empty value.
- The expressions "${name?value}" and "${name?{value}}"
      are replaced with "value" when "$name" is non-empty.
      The parameter name must contain only characters from the set [a-zA-Z0-9_].
      These forms are supported with Postfix versions >= 2.2 and >= 3.0,
      respectively.
- The expressions "${name:value}" and "${name:{value}}"
      are replaced with "value" when "$name" is empty. The
      parameter name must contain only characters from the set [a-zA-Z0-9_].
      These forms are supported with Postfix versions >= 2.2 and >= 3.0,
      respectively.
- The expression "${name?{value1}:{value2}}" is replaced with
      "value1" when "$name" is non-empty, and with
      "value2" when "$name" is empty. The "{}" is
      required for "value1", optional for "value2". The
      parameter name must contain only characters from the set [a-zA-Z0-9_].
      This form is supported with Postfix versions >= 3.0.
- The first item inside "${...}" may be a relational expression of
      the form: "{value3} == {value4}". Besides the "=="
      (equality) operator Postfix supports "!=" (inequality),
      "<", "<=", ">=", and
      ">". The comparison is numerical when both operands are all
      digits, otherwise the comparison is lexicographical. These forms are
      supported with Postfix versions >= 3.0.
- Each "value" is subject to recursive named parameter and
      relational expression evaluation, except where noted.
- Whitespace before or after each "{value}" is ignored.
- Specify "$$" to produce a single "$" character.
- The legacy form "$(...)" is equivalent to the preferred form
      "${...}".
 
  - When the same parameter is defined multiple times, only the last instance
      is remembered.
- Otherwise, the order of main.cf parameter definitions does not
    matter.
The remainder of this document is a description of all Postfix
    configuration parameters. Default values are shown after the parameter name
    in parentheses, and can be looked up with the "postconf -d"
    command.
Note: this is not an invitation to make changes to Postfix
    configuration parameters. Unnecessary changes can impair the operation of
    the mail system.
The recipient of undeliverable mail that cannot be returned to the sender. This
  feature is enabled with the notify_classes parameter.
The numerical Postfix SMTP server response code for an access(5) map
  "defer" action, including "defer_if_permit" or
  "defer_if_reject". Prior to Postfix 2.6, the response is hard-coded
  as "450".
Do not change this unless you have a complete understanding of RFC
    5321.
This feature is available in Postfix 2.6 and later.
The numerical Postfix SMTP server response code for an access(5) map
  "reject" action.
Do not change this unless you have a complete understanding of RFC
    5321.
The amount of time between verify(8) address verification database
  cleanup runs. This feature requires that the database supports the
  "delete" and "sequence" operators. Specify a zero interval
  to disable database cleanup.
After each database cleanup run, the verify(8) daemon logs
    the number of entries that were retained and dropped. A cleanup run is
    logged as "partial" when the daemon terminates early after
    "postfix reload", "postfix stop",
    or no requests for $max_idle seconds.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is h (hours).
This feature is available in Postfix 2.7.
Overrides the default_transport parameter setting for address verification
  probes.
This feature is available in Postfix 2.1 and later.
Overrides the local_transport parameter setting for address verification probes.
This feature is available in Postfix 2.1 and later.
Lookup table for persistent address verification status storage. The table is
  maintained by the verify(8) service, and is opened before the process
  releases privileges.
The lookup table is persistent by default (Postfix 2.7 and later).
    Specify an empty table name to keep the information in volatile memory which
    is lost after "postfix reload" or "postfix
    stop". This is the default with Postfix version 2.6 and
  earlier.
Specify a location in a file system that will not fill up. If the
    database becomes corrupted, the world comes to an end. To recover, delete
    (NOT: truncate) the file and do "postfix reload".
Postfix daemon processes do not use root privileges when opening
    this file (Postfix 2.5 and later). The file must therefore be stored under a
    Postfix-owned directory such as the data_directory. As a migration aid, an
    attempt to open the file under a non-Postfix directory is redirected to the
    Postfix-owned data_directory, and a warning is logged.
Examples:
address_verify_map = hash:/var/db/postfix/verify
address_verify_map = btree:/var/db/postfix/verify
This feature is available in Postfix 2.1 and later.
Enable caching of failed address verification probe results. When this feature
  is enabled, the cache may pollute quickly with garbage. When this feature is
  disabled, Postfix will generate an address probe for every lookup.
This feature is available in Postfix 2.1 and later.
The time after which a failed probe expires from the address verification cache.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is d
    (days).
This feature is available in Postfix 2.1 and later.
The time after which a failed address verification probe needs to be refreshed.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is h
    (hours).
This feature is available in Postfix 2.1 and later.
A safety limit that prevents address verification requests from overwhelming the
  Postfix queue. By default, the number of pending requests is limited to 1/4 of
  the active queue maximum size (qmgr_message_active_limit). The queue manager
  enforces the limit by tempfailing requests that exceed the limit. This affects
  only unknown addresses and inactive addresses that have expired, because the
  verify(8) daemon automatically refreshes an active address before it
  expires.
This feature is available in Postfix 3.1 and later.
How many times to query the verify(8) service for the completion of an
  address verification request in progress.
By default, the Postfix SMTP server polls the verify(8)
    service up to three times under non-overload conditions, and only once when
    under overload. With Postfix version 2.5 and earlier, the SMTP server always
    polls the verify(8) service up to three times by default.
Specify 1 to implement a crude form of greylisting, that is,
    always defer the first delivery request for a new address.
Examples:
# Postfix <= 2.6 default
address_verify_poll_count = 3
# Poor man's greylisting
address_verify_poll_count = 1
This feature is available in Postfix 2.1 and later.
The delay between queries for the completion of an address verification request
  in progress.
The default polling delay is 3 seconds.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
The time after which a successful probe expires from the address verification
  cache.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is d
    (days).
This feature is available in Postfix 2.1 and later.
The time after which a successful address verification probe needs to be
  refreshed. The address verification status is not updated when the probe fails
  (optimistic caching).
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is d
    (days).
This feature is available in Postfix 2.1 and later.
Overrides the relay_transport parameter setting for address verification probes.
This feature is available in Postfix 2.1 and later.
Overrides the relayhost parameter setting for address verification probes. This
  information can be overruled with the transport(5) table.
This feature is available in Postfix 2.1 and later.
The sender address to use in address verification probes; prior to Postfix 2.5
  the default was "postmaster". To avoid problems with address probes
  that are sent in response to address probes, the Postfix SMTP server excludes
  the probe sender address from all SMTPD access blocks.
Specify an empty value (address_verify_sender =) or <> if
    you want to use the null sender address. Beware, some sites reject mail from
    <>, even though RFCs require that such addresses be accepted.
Examples:
address_verify_sender = <>
address_verify_sender = postmaster@mydomain
This feature is available in Postfix 2.1 and later.
Overrides the sender_dependent_default_transport_maps parameter setting for
  address verification probes.
This feature is available in Postfix 2.7 and later.
Overrides the sender_dependent_relayhost_maps parameter setting for address
  verification probes.
This feature is available in Postfix 2.3 and later.
The time between changes in the time-dependent portion of address verification
  probe sender addresses. The time-dependent portion is appended to the
  localpart of the address specified with the address_verify_sender parameter.
  This feature is ignored when the probe sender addresses is the null sender,
  i.e. the address_verify_sender value is empty or <>.
Historically, the probe sender address was fixed. This has caused
    such addresses to end up on spammer mailing lists, and has resulted in
    wasted network and processing resources.
To enable time-dependent probe sender addresses, specify a
    non-zero time value. Specify a value of at least several hours, to avoid
    problems with senders that use greylisting. Avoid nice TTL values, to make
    the result less predictable.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
This feature is available in Postfix 2.9 and later.
The name of the verify(8) address verification service. This service
  maintains the status of sender and/or recipient address verification probes,
  and generates probes on request by other Postfix processes.
Overrides the transport_maps parameter setting for address verification probes.
This feature is available in Postfix 2.1 and later.
Overrides the virtual_transport parameter setting for address verification
  probes.
This feature is available in Postfix 2.1 and later.
The alias databases for local(8) delivery that are updated with
  "newaliases" or with "sendmail -bi".
This is a separate configuration parameter because not all the
    tables specified with $alias_maps have to be local files.
Examples:
alias_database = hash:/etc/aliases
alias_database = hash:/etc/mail/aliases
The alias databases that are used for local(8) delivery. See
  aliases(5) for syntax details. Specify zero or more
  "type:name" lookup tables, separated by whitespace or comma. Tables
  will be searched in the specified order until a match is found. Note: these
  lookups are recursive.
The default list is system dependent. On systems with NIS, the
    default is to search the local alias database, then the NIS alias
  database.
If you change the alias database, run "postalias
    /etc/aliases" (or wherever your system stores the mail alias file),
    or simply run "newaliases" to build the necessary DBM or DB
    file.
The local(8) delivery agent disallows regular expression
    substitution of $1 etc. in alias_maps, because that would open a security
    hole.
The local(8) delivery agent will silently ignore requests
    to use the proxymap(8) server within alias_maps. Instead it will open
    the table directly. Before Postfix version 2.2, the local(8) delivery
    agent will terminate with a fatal error.
Examples:
alias_maps = hash:/etc/aliases, nis:mail.aliases
alias_maps = hash:/etc/aliases
Restrict local(8) mail delivery to external commands. The default is to
  disallow delivery to "|command" in :include: files (see
  aliases(5) for the text that defines this terminology).
Specify zero or more of: alias, forward or
    include, in order to allow commands in aliases(5), .forward
    files or in :include: files, respectively.
Example:
allow_mail_to_commands = alias,forward,include
Restrict local(8) mail delivery to external files. The default is to
  disallow "/file/name" destinations in :include: files (see
  aliases(5) for the text that defines this terminology).
Specify zero or more of: alias, forward or
    include, in order to allow "/file/name" destinations in
    aliases(5), .forward files and in :include: files, respectively.
Example:
allow_mail_to_files = alias,forward,include
Allow a sender or recipient address to have `-' as the first character. By
  default, this is not allowed, to avoid accidents with software that passes
  email addresses via the command line. Such software would not be able to
  distinguish a malicious address from a bona fide command-line option. Although
  this can be prevented by inserting a "--" option terminator into the
  command line, this is difficult to enforce consistently and globally.
As of Postfix version 2.5, this feature is implemented by
    trivial-rewrite(8). With earlier versions this feature was
    implemented by qmgr(8) and was limited to recipient addresses
  only.
Enable the rewriting of the form "user%domain" to
  "user@domain". This is enabled by default.
Note: as of Postfix version 2.2, message header address rewriting
    happens only when one of the following conditions is true:
  - The message is received with the Postfix sendmail(1) command,
- The message is received from a network client that matches
      $local_header_rewrite_clients,
- The message is received from the network, and the
      remote_header_rewrite_domain parameter specifies a non-empty value.
    
 
To get the behavior before Postfix version 2.2, specify
    "local_header_rewrite_clients = static:all".
Example:
allow_percent_hack = no
When SRV record lookup fails or no SRV record exists, fall back to MX or IP
  address lookup as if SRV record lookup was not enabled.
This feature is available in Postfix 3.8 and later.
Forward mail with sender-specified routing (user[@%!]remote[@%!]site) from
  untrusted clients to destinations matching $relay_domains.
By default, this feature is turned off. This closes a nasty open
    relay loophole where a backup MX host can be tricked into forwarding junk
    mail to a primary MX host which then spams it out to the world.
This parameter also controls if non-local addresses with
    sender-specified routing can match Postfix access tables. By default, such
    addresses cannot match Postfix access tables, because the address is
    ambiguous.
A list of non-default Postfix configuration directories that may be specified
  with "-c config_directory" on the command line (in the case of
  sendmail(1), with the "-C" option), or via the MAIL_CONFIG
  environment parameter.
This list must be specified in the default Postfix main.cf file,
    and will be used by set-gid Postfix commands such as postqueue(1) and
    postdrop(1).
Specify absolute pathnames, separated by comma or space. Note:
    $name expansion is not supported.
Always add (Resent-) From:, To:, Date: or Message-ID: headers when not present.
  Postfix 2.6 and later add these headers only when clients match the
  local_header_rewrite_clients parameter setting. Earlier Postfix versions
  always add these headers; this may break DKIM signatures that cover
  non-existent headers. The undisclosed_recipients_header parameter setting
  determines whether a To: header will be added.
Optional address that receives a "blind carbon copy" of each message
  that is received by the Postfix mail system.
Note: with Postfix 2.3 and later the BCC address is added as if it
    was specified with NOTIFY=NONE. The sender will not be notified when the BCC
    address is undeliverable, as long as all down-stream software implements RFC
    3461.
Note: with Postfix 2.2 and earlier the sender will be notified
    when the BCC address is undeliverable.
Note: automatic BCC recipients are produced only for new mail. To
    avoid mailer loops, automatic BCC recipients are not generated after Postfix
    forwards mail internally, or after Postfix generates mail itself.
The time unit over which client connection rates and other rates are calculated.
This feature is implemented by the anvil(8) service which
    is available in Postfix version 2.2 and later.
The default interval is relatively short. Because of the high
    frequency of updates, the anvil(8) server uses volatile memory only.
    Thus, information is lost whenever the process terminates.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
How frequently the anvil(8) connection and rate limiting server logs peak
  usage information.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.2 and later.
With locally submitted mail, append the string "@$myorigin" to mail
  addresses without domain information. With remotely submitted mail, append the
  string "@$remote_header_rewrite_domain" instead.
Note 1: this feature is enabled by default and must not be turned
    off. Postfix does not support domain-less addresses.
Note 2: with Postfix version 2.2, message header address rewriting
    happens only when one of the following conditions is true:
  - The message is received with the Postfix sendmail(1) command,
- The message is received from a network client that matches
      $local_header_rewrite_clients,
- The message is received from the network, and the
      remote_header_rewrite_domain parameter specifies a non-empty value.
    
 
To get the behavior before Postfix version 2.2, specify
    "local_header_rewrite_clients = static:all".
With locally submitted mail, append the string ".$mydomain" to
  addresses that have no ".domain" information. With remotely
  submitted mail, append the string ".$remote_header_rewrite_domain"
  instead.
Note 1: When disabled (Postfix 3.0 and later), users will not be
    able to send mail to "user@partialdomainname" but will have to
    specify full domain names instead.
Note 2: with Postfix version 2.2, message header address rewriting
    happens only when one of the following conditions is true:
  - The message is received with the Postfix sendmail(1) command,
- The message is received from a network client that matches
      $local_header_rewrite_clients,
- The message is received from the network, and the
      remote_header_rewrite_domain parameter specifies a non-empty value.
    
 
To get the behavior before Postfix version 2.2, specify
    "local_header_rewrite_clients = static:all".
How long the postkick(1) command waits for a request to enter the Postfix
  daemon process input buffer before giving up.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
List of users who are authorized to flush the queue.
By default, all users are allowed to flush the queue. Access is
    always granted if the invoking user is the super-user or the $mail_owner
    user. Otherwise, the real UID of the process is looked up in the system
    password file, and access is granted only if the corresponding login name is
    on the access list. The username "unknown" is used for processes
    whose real UID is not found in the password file.
Specify a list of user names, "/file/name" or
    "type:table" patterns, separated by commas and/or whitespace. The
    list is matched left to right, and the search stops on the first match. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a name matches a lookup
    key (the lookup result is ignored). Continue long lines by starting the next
    line with whitespace. Specify "!pattern" to exclude a name from
    the list. The form "!/file/name" is supported only in Postfix
    version 2.4 and later.
This feature is available in Postfix 2.2 and later.
List of users who are authorized to view the queue.
By default, all users are allowed to view the queue. Access is
    always granted if the invoking user is the super-user or the $mail_owner
    user. Otherwise, the real UID of the process is looked up in the system
    password file, and access is granted only if the corresponding login name is
    on the access list. The username "unknown" is used for processes
    whose real UID is not found in the password file.
Specify a list of user names, "/file/name" or
    "type:table" patterns, separated by commas and/or whitespace. The
    list is matched left to right, and the search stops on the first match. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a name matches a lookup
    key (the lookup result is ignored). Continue long lines by starting the next
    line with whitespace. Specify "!pattern" to exclude a user name
    from the list. The form "!/file/name" is supported only in Postfix
    version 2.4 and later.
This feature is available in Postfix 2.2 and later.
List of users who are authorized to submit mail with the sendmail(1)
  command (and with the privileged postdrop(1) helper command).
By default, all users are allowed to submit mail. Otherwise, the
    real UID of the process is looked up in the system password file, and access
    is granted only if the corresponding login name is on the access list. The
    username "unknown" is used for processes whose real UID is not
    found in the password file. To deny mail submission access to all users
    specify an empty list.
Specify a list of user names, "/file/name" or
    "type:table" patterns, separated by commas and/or whitespace. The
    list is matched left to right, and the search stops on the first match. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a name matches a lookup
    key (the lookup result is ignored). Continue long lines by starting the next
    line with whitespace. Specify "!pattern" to exclude a user name
    from the list. The form "!/file/name" is supported only in Postfix
    version 2.4 and later.
Example:
authorized_submit_users = !www, static:all
This feature is available in Postfix 2.2 and later.
What remote SMTP clients are allowed to specify the XVERP command. This command
  requests that mail be delivered one recipient at a time with a per recipient
  return address.
By default, only trusted clients are allowed to specify XVERP.
This parameter was introduced with Postfix version 1.1. Postfix
    version 2.1 renamed this parameter to smtpd_authorized_verp_clients and
    changed the default to none.
Specify a list of network/netmask patterns, separated by commas
    and/or whitespace. The mask specifies the number of bits in the network part
    of a host address. You can also specify hostnames or .domain names (the
    initial dot causes the domain to match any name below it),
    "/file/name" or "type:table" patterns. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a table entry matches a
    lookup string (the lookup result is ignored). Continue long lines by
    starting the next line with whitespace. Specify "!pattern" to
    exclude an address or network block from the list. The form
    "!/file/name" is supported only in Postfix version 2.4 and
  later.
Note: IP version 6 address information must be specified inside []
    in the authorized_verp_clients value, and in files specified with
    "/file/name". IP version 6 addresses contain the ":"
    character, and would otherwise be confused with a "type:table"
    pattern.
Produce additional bounce(8) logfile records that can be read by Postfix
  versions before 2.0. The current and more extensible "name = value"
  format is needed in order to implement more sophisticated functionality.
This feature is available in Postfix 2.1 and later.
The per-table I/O buffer size for programs that create Berkeley DB hash or btree
  tables. Specify a byte count.
This feature is available in Postfix 2.0 and later.
The per-table I/O buffer size for programs that read Berkeley DB hash or btree
  tables. Specify a byte count.
This feature is available in Postfix 2.0 and later.
Where the Postfix SMTP client should deliver mail when it detects a "mail
  loops back to myself" error condition. This happens when the local MTA is
  the best SMTP mail exchanger for a destination not listed in $mydestination,
  $inet_interfaces, $proxy_interfaces, $virtual_alias_domains, or
  $virtual_mailbox_domains. By default, the Postfix SMTP client returns such
  mail as undeliverable.
Specify, for example, "best_mx_transport = local" to
    pass the mail from the Postfix SMTP client to the local(8) delivery
    agent. You can specify any message delivery "transport" or
    "transport:nexthop" that is defined in the master.cf file. See the
    transport(5) manual page for the syntax and meaning of
    "transport" or "transport:nexthop".
However, this feature is expensive because it ties up a Postfix
    SMTP client process while the local(8) delivery agent is doing its
    work. It is more efficient (for Postfix) to list all hosted domains in a
    table or database.
Whether or not to use the local biff service. This service sends "new
  mail" notifications to users who have requested new mail notification
  with the UNIX command "biff y".
For compatibility reasons this feature is on by default. On
    systems with lots of interactive users, the biff service can be a
    performance drain. Specify "biff = no" in main.cf to disable.
Optional lookup tables for content inspection as specified in the
  body_checks(5) manual page.
Note: with Postfix versions before 2.0, these rules inspect all
    content after the primary message headers.
How much text in a message body segment (or attachment, if you prefer to use
  that term) is subjected to body_checks inspection. The amount of text is
  limited to avoid scanning huge attachments.
This feature is available in Postfix 2.0 and later.
The recipient of postmaster notifications with the message headers of mail that
  Postfix did not deliver and of SMTP conversation transcripts of mail that
  Postfix did not receive. This feature is enabled with the notify_classes
  parameter.
Consider a bounce message as undeliverable, when delivery fails with a temporary
  error, and the time in the queue has reached the bounce_queue_lifetime limit.
  By default, this limit is the same as for regular mail.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is d (days).
Specify 0 when mail delivery should be tried only once.
This feature is available in Postfix 2.1 and later.
The name of the bounce(8) service. This service maintains a record of
  failed delivery attempts and generates non-delivery notifications.
This feature is available in Postfix 2.0 and later.
The maximal amount of original message text that is sent in a non-delivery
  notification. Specify a byte count. A message is returned as either
  message/rfc822 (the complete original) or as text/rfc822-headers (the headers
  only). With Postfix version 2.4 and earlier, a message is always returned as
  message/rfc822 and is truncated when it exceeds the size limit.
Notes:
  - If you increase this limit, then you should increase the
      mime_nesting_limit value proportionally.
- Be careful when making changes. Excessively large values will result in
      the loss of non-delivery notifications, when a bounce message size exceeds
      a local or remote MTA's message size limit.
    
 
Pathname of a configuration file with bounce message templates. These override
  the built-in templates of delivery status notification (DSN) messages for
  undeliverable mail, delayed mail, successful delivery, or delivery
  verification. The bounce(5) manual page describes how to edit and test
  template files.Template message body text may contain $name references to Postfix
    configuration parameters. The result of $name expansion can be previewed
    with "postconf -b file_name" before the file is
    placed into the Postfix configuration directory.
This feature is available in Postfix 2.3 and later.
Enable interoperability with remote SMTP clients that implement an obsolete
  version of the AUTH command (RFC 4954). Examples of such clients are MicroSoft
  Outlook Express version 4 and MicroSoft Exchange version 5.0.
Specify "broken_sasl_auth_clients = yes" to have Postfix
    advertise AUTH support in a non-standard way.
What addresses are subject to canonical_maps address mapping. By default,
  canonical_maps address mapping is applied to envelope sender and recipient
  addresses, and to header sender and header recipient addresses.
Specify one or more of: envelope_sender, envelope_recipient,
    header_sender, header_recipient
This feature is available in Postfix 2.2 and later.
Optional address mapping lookup tables for message headers and envelopes. The
  mapping is applied to both sender and recipient addresses, in both envelopes
  and in headers, as controlled with the canonical_classes parameter. This is
  typically used to clean up dirty addresses from legacy mail systems, or to
  replace login names by Firstname.Lastname. The table format and lookups are
  documented in canonical(5). For an overview of Postfix address
  manipulations see the ADDRESS_REWRITING_README document.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found. Note: these lookups are recursive.
If you use this feature, run "postmap
    /etc/postfix/canonical" to build the necessary DBM or DB file after
    every change. The changes will become visible after a minute or so. Use
    "postfix reload" to eliminate the delay.
Note: with Postfix version 2.2, message header address mapping
    happens only when message header address rewriting is enabled:
  - The message is received with the Postfix sendmail(1) command,
- The message is received from a network client that matches
      $local_header_rewrite_clients,
- The message is received from the network, and the
      remote_header_rewrite_domain parameter specifies a non-empty value.
    
 
To get the behavior before Postfix version 2.2, specify
    "local_header_rewrite_clients = static:all".
Examples:
canonical_maps = dbm:/etc/postfix/canonical
canonical_maps = hash:/etc/postfix/canonical
The name of the cleanup(8) service. This service rewrites addresses into
  the standard form, and performs canonical(5) address mapping and
  virtual(5) aliasing.
This feature is available in Postfix 2.0 and later.
The location of all postfix administrative commands.
The local(8) delivery agent working directory for delivery to external
  commands. Failure to change directory causes the delivery to be deferred.
The command_execution_directory value is not subject to Postfix
    configuration parameter $name expansion. Instead, the following $name
    expansions are done on command_execution_directory before the directory is
    used. Expansion happens in the context of the delivery request. The result
    of $name expansion is filtered with the character set that is specified with
    the execution_directory_expansion_filter parameter.
  - $user
- The recipient's username.
    
 
- $shell
- The recipient's login shell pathname.
    
 
- $home
- The recipient's home directory.
    
 
- $recipient
- The full recipient address.
    
 
- $extension
- The optional recipient address extension.
    
 
- $domain
- The recipient domain.
    
 
- $local
- The entire recipient localpart.
    
 
- $recipient_delimiter
- The address extension delimiter that was found in the recipient address
      (Postfix 2.11 and later), or the system-wide recipient address extension
      delimiter (Postfix 2.10 and earlier).
    
 
- ${name?value}
- ${name?{value}} (Postfix >= 3.0)
- Expands to value when $name is non-empty.
    
 
- ${name:value}
- ${name:{value}} (Postfix >= 3.0)
- Expands to value when $name is empty.
    
 
- ${name?{value1}:{value2}} (Postfix >= 3.0)
- Expands to value1 when $name is non-empty, value2
      otherwise.
    
 
Instead of $name you can also specify ${name} or $(name).
This feature is available in Postfix 2.2 and later.
Restrict the characters that the local(8) delivery agent allows in $name
  expansions of $mailbox_command and $command_execution_directory. Characters
  outside the allowed set are replaced by underscores.
Time limit for delivery to external commands. This limit is used by the
  local(8) delivery agent, and is the default time limit for delivery by
  the pipe(8) delivery agent.
Note: if you set this time limit to a large value you must update
    the global ipc_timeout parameter as well.
A safety net that causes Postfix to run with backwards-compatible default
  settings after an upgrade to a newer Postfix version.
With backwards compatibility turned on (the main.cf
    compatibility_level value is less than the Postfix built-in value), Postfix
    looks for settings that are left at their implicit default value, and logs a
    message when a backwards-compatible default setting is required.
  
using backwards-compatible default setting name=value
    to [accept a specific client request]
using backwards-compatible default setting name=value
    to [enable specific Postfix behavior]
See COMPATIBILITY_README for specific message details. If such a
    message is logged in the context of a legitimate request, the system
    administrator should make the backwards-compatible setting permanent in
    main.cf or master.cf, for example:
  
# postconf name=value
# postfix reload
When no more backwards-compatible settings need to be made
    permanent, the administrator should turn off backwards compatibility by
    updating the compatibility_level setting in main.cf:
  
# postconf compatibility_level=N
# postfix reload
For N specify the number that is logged in your
    postfix(1) warning message:
  
warning: To disable backwards compatibility use "postconf
    compatibility_level=N" and "postfix reload"
Starting with Postfix version 3.6, the compatibility level in the
    above warning message is the Postfix version that introduced the last
    incompatible change. The level is formatted as major.minor.patch,
    where patch is usually omitted and defaults to zero. Earlier
    compatibility levels are 0, 1 and 2.
NOTE: this also introduces support for the "<level",
    "<=level", and other operators to compare compatibility levels.
    With the standard operators "<", "<=", etc.,
    compatibility level "3.10" would be smaller than "3.9"
    which is undesirable.
This feature is available in Postfix 3.0 and later.
The default location of the Postfix main.cf and master.cf configuration files.
  This can be overruled via the following mechanisms:
  - The MAIL_CONFIG environment variable (daemon processes and commands).
- The "-c" command-line option (commands only).
    
 
With Postfix commands that run with set-gid privileges, a
    config_directory override either requires root privileges, or it requires
    that the directory is listed with the alternate_config_directories parameter
    in the default main.cf file.
After sending a "your message is delayed" notification, inform the
  sender when the delay clears up. This can result in a sudden burst of
  notifications at the end of a prolonged network outage, and is therefore
  disabled by default.
See also: delay_warning_time.
This feature is available in Postfix 3.0 and later.
Time limit for connection cache connect, send or receive operations. The time
  limit is enforced in the client.
This feature is available in Postfix 2.3 and later.
The name of the scache(8) connection cache service. This service
  maintains a limited pool of cached sessions.
This feature is available in Postfix 2.2 and later.
How frequently the scache(8) server logs usage statistics with connection
  cache hit and miss rates for logical destinations and for physical endpoints.
The maximal time-to-live value that the scache(8) connection cache server
  allows. Requests that specify a larger TTL will be stored with the maximum
  allowed TTL. The purpose of this additional control is to protect the
  infrastructure against careless people. The cache TTL is already bounded by
  $max_idle.
After the message is queued, send the entire message to the specified
  transport:destination. The transport name specifies the first
  field of a mail delivery agent definition in master.cf; the syntax of the
  next-hop destination is described in the manual page of the
  corresponding delivery agent. More information about external content filters
  is in the Postfix FILTER_README file.
Notes:
  - This setting has lower precedence than a FILTER action that is specified
      in an access(5), header_checks(5) or body_checks(5)
      table.
- The meaning of an empty next-hop filter destination is version
      dependent. Postfix 2.7 and later will use the recipient domain; earlier
      versions will use $myhostname. Specify "default_filter_nexthop =
      $myhostname" for compatibility with Postfix 2.6 or earlier, or
      specify a content_filter value with an explicit next-hop
      destination.
    
 
Search path for Cyrus SASL application configuration files, currently used only
  to locate the $smtpd_sasl_path.conf file. Specify zero or more directories
  separated by a colon character, or an empty value to use Cyrus SASL's built-in
  search path.This feature is available in Postfix 2.5 and later when compiled
    with Cyrus SASL 2.1.22 or later.
The directory with Postfix support programs and daemon programs. These should
  not be invoked directly by humans. The directory must be owned by root.
How a Postfix daemon process handles errors while opening lookup tables: gradual
  degradation or immediate termination.
  -  no  (default)
- Gradual degradation: a daemon process logs a message of type
      "error" and continues execution with reduced functionality.
      Features that do not depend on the unavailable table will work normally,
      while features that depend on the table will result in a type
      "warning" message.
    
 When the notify_classes parameter value contains the "data" class,
      the Postfix SMTP server and client will report transcripts of sessions
      with an error because a table is unavailable.
 
-  yes  (historical behavior)
- Immediate termination: a daemon process logs a type "fatal"
      message and terminates immediately. This option reduces the number of
      possible code paths through Postfix, and may therefore be slightly more
      secure than the default.
    
 
For the sake of sanity, the number of type "error"
    messages is limited to 13 over the lifetime of a daemon process.
This feature is available in Postfix 2.9 and later.
How much time a Postfix daemon process may take to handle a request before it is
  terminated by a built-in watchdog timer.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The directory with Postfix-writable data files (for example: caches,
  pseudo-random numbers). This directory must be owned by the mail_owner
  account, and must not be shared with non-Postfix software.
This feature is available in Postfix 2.5 and later.
The increment in verbose logging level when a nexthop destination, remote client
  or server name or network address matches a pattern given with the
  debug_peer_list parameter.
Per-nexthop debug logging is available in Postfix 3.6 and
  later.
Optional list of nexthop destination, remote client or server name or network
  address patterns that, if matched, cause the verbose logging level to increase
  by the amount specified in $debug_peer_level.
Per-nexthop debug logging is available in Postfix 3.6 and
  later.
Specify domain names, network/netmask patterns,
    "/file/name" patterns or "type:table" lookup tables. The
    right-hand side result from "type:table" lookups is ignored.
Pattern matching of domain names is controlled by the presence or
    absence of "debug_peer_list" in the
    parent_domain_matches_subdomains parameter value.
Examples:
debug_peer_list = 127.0.0.1
debug_peer_list = example.com
The external command to execute when a Postfix daemon program is invoked with
  the -D option.
Use "command .. & sleep 5" so that the debugger can
    attach before the process marches on. If you use an X-based debugger, be
    sure to set up your XAUTHORITY environment variable before starting
  Postfix.
Note: the command is subject to $name expansion, before it is
    passed to the default command interpreter. Specify "$$" to produce
    a single "$" character.
Example:
debugger_command =
    PATH=/usr/bin:/usr/X11R6/bin
    ddd $daemon_directory/$process_name $process_id & sleep 5
The default database type for use in newaliases(1), postalias(1)
  and postmap(1) commands. On many UNIX systems the default type is
  either dbm or hash. The default setting is frozen when the
  Postfix system is built.
Examples:
default_database_type = hash
default_database_type = dbm
How often the Postfix queue manager's scheduler is allowed to preempt delivery
  of one message with another.
Each transport maintains a so-called "available delivery slot
    counter" for each message. One message can be preempted by another one
    when the other message can be delivered using no more delivery slots (i.e.,
    invocations of delivery agents) than the current message counter has
    accumulated (or will eventually accumulate - see about slot loans below).
    This parameter controls how often the counter is incremented - it happens
    after each default_delivery_slot_cost recipients have been delivered.
The cost of 0 is used to disable the preempting scheduling
    completely. The minimum value the scheduling algorithm can use is 2 - use it
    if you want to maximize the message throughput rate. Although there is no
    maximum, it doesn't make much sense to use values above say 50.
The only reason why the value of 2 is not the default is the way
    this parameter affects the delivery of mailing-list mail. In the worst case,
    delivery can take somewhere between (cost+1/cost) and (cost/cost-1) times
    more than if the preemptive scheduler was disabled. The default value of 5
    turns out to provide reasonable message response times while making sure the
    mailing-list deliveries are not extended by more than 20-25 percent even in
    the worst case.
Use transport_delivery_slot_cost to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
Examples:
default_delivery_slot_cost = 0
default_delivery_slot_cost = 2
The default value for transport-specific _delivery_slot_discount settings.
This parameter speeds up the moment when a message preemption can
    happen. Instead of waiting until the full amount of delivery slots required
    is available, the preemption can happen when
    transport_delivery_slot_discount percent of the required amount plus
    transport_delivery_slot_loan still remains to be accumulated. Note
    that the full amount will still have to be accumulated before another
    preemption can take place later.
Use transport_delivery_slot_discount to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
The default value for transport-specific _delivery_slot_loan settings.
This parameter speeds up the moment when a message preemption can
    happen. Instead of waiting until the full amount of delivery slots required
    is available, the preemption can happen when
    transport_delivery_slot_discount percent of the required amount plus
    transport_delivery_slot_loan still remains to be accumulated. Note that the
    full amount will still have to be accumulated before another preemption can
    take place later.
Use transport_delivery_slot_loan to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
Optional filter to replace the delivery status code or explanatory text of
  successful or unsuccessful deliveries. This does not allow the replacement of
  a successful status code (2.X.X) with an unsuccessful status code (4.X.X or
  5.X.X) or vice versa.
Note: the (smtp|lmtp)_delivery_status_filter is applied only once
    per recipient: when delivery is successful, when delivery is rejected with
    5XX, or when there are no more alternate MX or A destinations. Use
    smtp_reply_filter or lmtp_reply_filter to inspect responses for all delivery
    attempts.
The following parameters can be used to implement a filter for
    specific delivery agents: lmtp_delivery_status_filter,
    local_delivery_status_filter, pipe_delivery_status_filter,
    smtp_delivery_status_filter or virtual_delivery_status_filter. These
    parameters support the same filter syntax as described here.
Specify zero or more "type:table" lookup table names,
    separated by comma or whitespace. For each successful or unsuccessful
    delivery to a recipient, the tables are queried in the specified order with
    one line of text that is structured as follows:
  
  enhanced-status-code SPACE explanatory-text
  
The first table match wins. The lookup result must have the same
    structure as the query, a successful status code (2.X.X) must be replaced
    with a successful status code, an unsuccessful status code (4.X.X or 5.X.X)
    must be replaced with an unsuccessful status code, and the explanatory text
    field must be non-empty. Other results will result in a warning.
Example 1: convert specific soft TLS errors into hard errors, by
    overriding the first number in the enhanced status code.
  
/etc/postfix/main.cf:
    smtp_delivery_status_filter = pcre:/etc/postfix/smtp_dsn_filter
  
/etc/postfix/smtp_dsn_filter:
    /^4(\.\d+\.\d+ TLS is required, but host \S+ refused to start TLS: .+)/
        5$1
    /^4(\.\d+\.\d+ TLS is required, but was not offered by host .+)/
        5$1
    # Do not change the following into hard bounces. They may
    # result from a local configuration problem.
    # 4.\d+.\d+ TLS is required, but our TLS engine is unavailable
    # 4.\d+.\d+ TLS is required, but unavailable
    # 4.\d+.\d+ Cannot start TLS: handshake failure
Example 2: censor the per-recipient delivery status text so that
    it does not reveal the destination command or filename when a remote sender
    requests confirmation of successful delivery.
  
/etc/postfix/main.cf:
    local_delivery_status_filter = pcre:/etc/postfix/local_dsn_filter
  
/etc/postfix/local_dsn_filter:
    /^(2\S+ delivered to file).+/    $1
    /^(2\S+ delivered to command).+/ $1
Notes:
  - This feature will NOT override the soft_bounce safety net.
- This feature will change the enhanced status code and text that is logged
      to the maillog file, and that is reported to the sender in delivery
      confirmation or non-delivery notifications.
    
 
This feature is available in Postfix 3.0 and later.
How many pseudo-cohorts must suffer connection or handshake failure before a
  specific destination is considered unavailable (and further delivery is
  suspended). Specify zero to disable this feature. A destination's
  pseudo-cohort failure count is reset each time a delivery completes without
  connection or handshake failure for that specific destination.
A pseudo-cohort is the number of deliveries equal to a
    destination's delivery concurrency.
Use transport_destination_concurrency_failed_cohort_limit
    to specify a transport-specific override, where transport is the
    master.cf name of the message delivery transport.
This feature is available in Postfix 2.5. The default setting is
    compatible with earlier Postfix versions.
The default maximal number of parallel deliveries to the same destination. This
  is the default limit for delivery via the lmtp(8), pipe(8),
  smtp(8) and virtual(8) delivery agents. With a per-destination
  recipient limit > 1, a destination is a domain, otherwise it is a
  recipient.
Use transport_destination_concurrency_limit to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
The per-destination amount of delivery concurrency negative feedback, after a
  delivery completes with a connection or handshake failure. Feedback values are
  in the range 0..1 inclusive. With negative feedback, concurrency is
  decremented at the beginning of a sequence of length 1/feedback. This is
  unlike positive feedback, where concurrency is incremented at the end of a
  sequence of length 1/feedback.
As of Postfix version 2.5, negative feedback cannot reduce
    delivery concurrency to zero. Instead, a destination is marked dead (further
    delivery suspended) after the failed pseudo-cohort count reaches
    $default_destination_concurrency_failed_cohort_limit (or
    $transport_destination_concurrency_failed_cohort_limit). To make the
    scheduler completely immune to connection or handshake failures, specify a
    zero feedback value and a zero failed pseudo-cohort limit.
Specify one of the following forms:
  - number 
- number / number 
- Constant feedback. The value must be in the range 0..1 inclusive. The
      default setting of "1" is compatible with Postfix versions
      before 2.5, where a destination's delivery concurrency is throttled down
      to zero (and further delivery suspended) after a single failed
      pseudo-cohort.
    
 
- number / concurrency 
- Variable feedback of "number / (delivery concurrency)".
      The number must be in the range 0..1 inclusive. With number
      equal to "1", a destination's delivery concurrency is
      decremented by 1 after each failed pseudo-cohort.
    
 
A pseudo-cohort is the number of deliveries equal to a
    destination's delivery concurrency.
Use transport_destination_concurrency_negative_feedback to
    specify a transport-specific override, where transport is the
    master.cf name of the message delivery transport.
This feature is available in Postfix 2.5. The default setting is
    compatible with earlier Postfix versions.
The per-destination amount of delivery concurrency positive feedback, after a
  delivery completes without connection or handshake failure. Feedback values
  are in the range 0..1 inclusive. The concurrency increases until it reaches
  the per-destination maximal concurrency limit. With positive feedback,
  concurrency is incremented at the end of a sequence with length 1/feedback.
  This is unlike negative feedback, where concurrency is decremented at the
  start of a sequence of length 1/feedback.
Specify one of the following forms:
  - number 
- number / number 
- Constant feedback. The value must be in the range 0..1 inclusive. The
      default setting of "1" is compatible with Postfix versions
      before 2.5, where a destination's delivery concurrency doubles after each
      successful pseudo-cohort.
    
 
- number / concurrency 
- Variable feedback of "number / (delivery concurrency)".
      The number must be in the range 0..1 inclusive. With number
      equal to "1", a destination's delivery concurrency is
      incremented by 1 after each successful pseudo-cohort.
    
 
A pseudo-cohort is the number of deliveries equal to a
    destination's delivery concurrency.
Use transport_destination_concurrency_positive_feedback to
    specify a transport-specific override, where transport is the
    master.cf name of the message delivery transport.
This feature is available in Postfix 2.5 and later.
The default amount of delay that is inserted between individual message
  deliveries to the same destination and over the same message delivery
  transport. Specify a non-zero value to rate-limit those message deliveries to
  at most one per $default_destination_rate_delay.
The resulting behavior depends on the value of the corresponding
    per-destination recipient limit.
  - With a corresponding per-destination recipient limit > 1, the rate
      delay specifies the time between deliveries to the same domain.
      Different domains are delivered in parallel, subject to the process limits
      specified in master.cf.
- With a corresponding per-destination recipient limit equal to 1, the rate
      delay specifies the time between deliveries to the same recipient.
      Different recipients are delivered in parallel, subject to the process
      limits specified in master.cf.
    
 
To enable the delay, specify a non-zero time value (an integral
    value plus an optional one-letter suffix that specifies the time unit).
Time units: s (seconds), m (minutes), h (hours), d (days), w
    (weeks). The default time unit is s (seconds).
NOTE: the delay is enforced by the queue manager. The delay timer
    state does not survive "postfix reload" or
    "postfix stop".
Use transport_destination_rate_delay to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
NOTE: with a non-zero _destination_rate_delay, specify a
    transport_destination_concurrency_failed_cohort_limit of 10 or more
    to prevent Postfix from deferring all mail for the same destination after
    only one connection or handshake error.
This feature is available in Postfix 2.5 and later.
The default maximal number of recipients per message delivery. This is the
  default limit for delivery via the lmtp(8), pipe(8),
  smtp(8) and virtual(8) delivery agents.
Setting this parameter to a value of 1 affects email deliveries as
    follows:
  - It changes the meaning of the corresponding per-destination concurrency
      limit, from concurrency of deliveries to the same domain
      into concurrency of deliveries to the same recipient. Different
      recipients are delivered in parallel, subject to the process limits
      specified in master.cf.
- It changes the meaning of the corresponding per-destination rate delay,
      from the delay between deliveries to the same domain into
      the delay between deliveries to the same recipient. Again,
      different recipients are delivered in parallel, subject to the process
      limits specified in master.cf.
- It changes the meaning of other corresponding per-destination settings in
      a similar manner, from settings for delivery to the same domain
      into settings for delivery to the same recipient.
    
 
Use transport_destination_recipient_limit to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
The default value for the extra per-transport limit imposed on the number of
  in-memory recipients. This extra recipient space is reserved for the cases
  when the Postfix queue manager's scheduler preempts one message with another
  and suddenly needs some extra recipient slots for the chosen message in order
  to avoid performance degradation.
Use transport_extra_recipient_limit to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
When a content_filter or FILTER request specifies no explicit next-hop
  destination, use $default_filter_nexthop instead; when that value is empty,
  use the domain in the recipient address. Specify "default_filter_nexthop
  = $myhostname" for compatibility with Postfix version 2.6 and earlier, or
  specify an explicit next-hop destination with each content_filter value or
  FILTER action.
This feature is available in Postfix 2.7 and later.
How many recipients a message must have in order to invoke the Postfix queue
  manager's scheduling algorithm at all. Messages which would never accumulate
  at least this many delivery slots (subject to slot cost parameter as well) are
  never preempted.
Use transport_minimum_delivery_slots to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
The default rights used by the local(8) delivery agent for delivery to an
  external file or command. These rights are used when delivery is requested
  from an aliases(5) file that is owned by root, or when delivery
  is done on behalf of root. DO NOT SPECIFY A PRIVILEGED USER
  OR THE POSTFIX OWNER.
The default maximal number of Postfix child processes that provide a given
  service. This limit can be overruled for specific services in the master.cf
  file.
The default Postfix SMTP server response template for a request that is rejected
  by an RBL-based restriction. This template can be overruled by specific
  entries in the optional rbl_reply_maps lookup table.
This feature is available in Postfix 2.0 and later.
The template does not support Postfix configuration parameter
    $name substitution. Instead, it supports exactly one level of $name
    substitution for the following attributes:
  - $client
- The client hostname and IP address, formatted as name[address].
    
 
- $client_address
- The client IP address.
    
 
- $client_name
- The client hostname or "unknown". See
      reject_unknown_client_hostname for more details.
    
 
- $reverse_client_name
- The client hostname from address->name lookup, or "unknown".
      See reject_unknown_reverse_client_hostname for more details.
    
 
- $helo_name
- The hostname given in HELO or EHLO command or empty string.
    
 
- $rbl_class
- The denylisted entity type: Client host, Helo command, Sender address, or
      Recipient address.
    
 
- $rbl_code
- The numerical SMTP response code, as specified with the
      maps_rbl_reject_code configuration parameter. Note: The numerical SMTP
      response code is required, and must appear at the start of the reply. With
      Postfix version 2.3 and later this information may be followed by an RFC
      3463 enhanced status code.
    
 
- $rbl_domain
- The RBL domain where $rbl_what is denylisted.
    
 
- $rbl_reason
- The reason why $rbl_what is denylisted, or an empty string.
    
 
- $rbl_what
- The entity that is denylisted (an IP address, a hostname, a domain name,
      or an email address whose domain was denylisted).
    
 
- $recipient
- The recipient address or <> in case of the null address.
    
 
- $recipient_domain
- The recipient domain or empty string.
    
 
- $recipient_name
- The recipient address localpart or <> in case of null address.
    
 
- $sender
- The sender address or <> in case of the null address.
    
 
- $sender_domain
- The sender domain or empty string.
    
 
- $sender_name
- The sender address localpart or <> in case of the null address.
    
 
- ${name?value}
- ${name?{value}} (Postfix >= 3.0)
- Expands to value when $name is non-empty.
    
 
- ${name:value}
- ${name:{value}} (Postfix >= 3.0)
- Expands to value when $name is empty.
    
 
- ${name?{value1}:{value2}} (Postfix >= 3.0)
- Expands to value1 when $name is non-empty, value2
      otherwise.
    
 
Instead of $name you can also specify ${name} or $(name).
Note: when an enhanced status code is specified in an RBL reply
    template, it is subject to modification. The following transformations are
    needed when the same RBL reply template is used for client, helo, sender, or
    recipient access restrictions.
  - When rejecting a sender address, the Postfix SMTP server will transform a
      recipient DSN status (e.g., 4.1.1-4.1.6) into the corresponding sender DSN
      status, and vice versa.
- When rejecting non-address information (such as the HELO command argument
      or the client hostname/address), the Postfix SMTP server will transform a
      sender or recipient DSN status into a generic non-address DSN status
      (e.g., 4.0.0).
    
 
The default per-transport upper limit on the number of in-memory recipients.
  These limits take priority over the global qmgr_message_recipient_limit after
  the message has been assigned to the respective transports. See also
  default_extra_recipient_limit and qmgr_message_recipient_minimum.Use transport_recipient_limit to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
The default per-transport maximum delay between refilling recipients. When not
  all message recipients fit into memory at once, keep loading more of them at
  least once every this many seconds. This is used to make sure the recipients
  are refilled in a timely manner even when $default_recipient_refill_limit is
  too high for too slow deliveries.
Use transport_recipient_refill_delay to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
This feature is available in Postfix 2.4 and later.
The default per-transport limit on the number of recipients refilled at once.
  When not all message recipients fit into memory at once, keep loading more of
  them in batches of at least this many at a time. See also
  $default_recipient_refill_delay, which may result in recipient batches lower
  than this when this limit is too high for too slow deliveries.
Use transport_recipient_refill_limit to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport.
This feature is available in Postfix 2.4 and later.
The default mail delivery transport and next-hop destination for destinations
  that do not match $mydestination, $inet_interfaces, $proxy_interfaces,
  $virtual_alias_domains, $virtual_mailbox_domains, or $relay_domains. This
  information can be overruled with the sender_dependent_default_transport_maps
  parameter and with the transport(5) table.
In order of decreasing precedence, the nexthop destination is
    taken from $sender_dependent_default_transport_maps, $default_transport,
    $sender_dependent_relayhost_maps, $relayhost, or from the recipient
  domain.
Specify a string of the form transport:nexthop, where
    transport is the name of a mail delivery transport defined in
    master.cf. The :nexthop destination is optional; its syntax is
    documented in the manual page of the corresponding delivery agent. In the
    case of SMTP or LMTP, specify one or more destinations separated by comma or
    whitespace (with Postfix 3.5 and later).
Example:
default_transport = uucp:relayhostname
The default amount of delay that is inserted between individual message
  deliveries over the same message delivery transport, regardless of
  destination. Specify a non-zero value to rate-limit those message deliveries
  to at most one per $default_transport_rate_delay.
Use transport_transport_rate_delay to specify a
    transport-specific override, where the initial transport is the
    master.cf name of the message delivery transport.
Example: throttle outbound SMTP mail to at most 3 deliveries per
    minute.
/etc/postfix/main.cf:
    smtp_transport_rate_delay = 20s
To enable the delay, specify a non-zero time value (an integral
    value plus an optional one-letter suffix that specifies the time unit).
Time units: s (seconds), m (minutes), h (hours), d (days), w
    (weeks). The default time unit is s (seconds).
NOTE: the delay is enforced by the queue manager.
This feature is available in Postfix 3.1 and later.
The two default VERP delimiter characters. These are used when no explicit
  delimiters are specified with the SMTP XVERP command or with the
  "sendmail -XV" command-line option (Postfix 2.2 and earlier:
  -V). Specify characters that are allowed by the verp_delimiter_filter
  setting.
This feature is available in Postfix 1.1 and later.
The numerical Postfix SMTP server response code when a remote SMTP client
  request is rejected by the "defer" restriction.
Do not change this unless you have a complete understanding of RFC
    5321.
The name of the defer service. This service is implemented by the
  bounce(8) daemon and maintains a record of failed delivery attempts and
  generates non-delivery notifications.
This feature is available in Postfix 2.0 and later.
The names of message delivery transports that should not deliver mail unless
  someone issues "sendmail -q" or equivalent. Specify zero or
  more mail delivery transport names that appear in the first field of
  master.cf.
Example:
defer_transports = smtp
The maximal number of digits after the decimal point when logging sub-second
  delay values. Specify a number in the range 0..6.
Large delay values are rounded off to an integral number of
    seconds; delay values below the delay_logging_resolution_limit are logged as
    "0", and delay values under 100s are logged with at most two-digit
    precision.
The format of the "delays=a/b/c/d" logging is as
    follows:
  - a = time from message arrival to last active queue entry
- b = time from last active queue entry to connection setup
- c = time in connection setup, including DNS, EHLO and STARTTLS
- d = time in message transmission
    
 
This feature is available in Postfix 2.3 and later.
The recipient of postmaster notifications with the message headers of mail that
  cannot be delivered within $delay_warning_time time units.
See also: delay_warning_time, notify_classes.
The time after which the sender receives a copy of the message headers of mail
  that is still queued. The confirm_delay_cleared parameter controls sender
  notification when the delay clears up.
To enable this feature, specify a non-zero time value (an integral
    value plus an optional one-letter suffix that specifies the time unit).
Time units: s (seconds), m (minutes), h (hours), d (days), w
    (weeks). The default time unit is h (hours).
See also: delay_notice_recipient, notify_classes,
    confirm_delay_cleared.
The maximal number of attempts to acquire an exclusive lock on a mailbox file or
  bounce(8) logfile.
The time between attempts to acquire an exclusive lock on a mailbox file or
  bounce(8) logfile.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
Make the queue manager's feedback algorithm verbose for performance analysis
  purposes.
This feature is available in Postfix 2.5 and later.
Automatically detect 8BITMIME body content by looking at
  Content-Transfer-Encoding: message headers; historically, this behavior was
  hard-coded to be "always on".
This feature is available in Postfix 2.5 and later.
Disable DNS lookups in the Postfix SMTP and LMTP clients. When disabled, hosts
  are looked up with the getaddrinfo() system library routine which normally
  also looks in /etc/hosts. As of Postfix 2.11, this parameter is deprecated;
  use smtp_dns_support_level instead.
DNS lookups are enabled by default.
Turn off MIME processing while receiving mail. This means that no special
  treatment is given to Content-Type: message headers, and that all text after
  the initial message headers is considered to be part of the message body.
This feature is available in Postfix 2.0 and later.
Mime input processing is enabled by default, and is needed in
    order to recognize MIME headers in message content.
Disable the conversion of 8BITMIME format to 7BIT format. Mime output conversion
  is needed when the destination does not advertise 8BITMIME support.
This feature is available in Postfix 2.0 and later.
Disable sending one bounce report per recipient.
The default, one per recipient, is what ezmlm needs.
This feature is available in Postfix 1.1 and later.
Disable the SMTP VRFY command. This stops some techniques used to harvest email
  addresses.
Example:
disable_vrfy_command = no
Enable a workaround for future libc incompatibility. The Postfix implementation
  of RFC 2308 negative reply caching relies on the promise that res_query() and
  res_search() invoke res_send(), which returns the server response in an
  application buffer even if the requested record does not exist. If this
  promise is broken, specify "yes" to enable a workaround for DNS
  reputation lookups.
This feature is available in Postfix 3.1 and later.
A debugging aid to artificially delay DNS responses.
This feature is available in Postfix 2.8.
The name of the dnsblog(8) service entry in master.cf. This service
  performs DNS allow/denylist lookups.
This feature is available in Postfix 2.8 and later.
The DNS query type (default: "ns") and DNS query name (default:
  ".") that Postfix may use to determine whether DNSSEC validation is
  available.
Background: DNSSEC validation is needed for Postfix DANE support;
    this ensures that Postfix receives TLSA records with secure TLS server
    certificate info. When DNSSEC validation is unavailable, mail deliveries
    using opportunistic DANE will not be protected by server certificate
    info in TLSA records, and mail deliveries using mandatory DANE will
    not be made at all.
By default, a Postfix process will send a DNSSEC probe after 1)
    the process made a DNS query that requested DNSSEC validation, 2) the
    process did not receive a DNSSEC validated response to this query or to an
    earlier query, and 3) the process did not already send a DNSSEC probe.
When the DNSSEC probe has no response, or when the response is not
    DNSSEC validated, Postfix logs a warning that DNSSEC validation may be
    unavailable.
Example:
warning: DNSSEC validation may be unavailable
warning: reason: dnssec_probe 'ns:.' received a response that is not DNSSEC validated
warning: reason: dnssec_probe 'ns:.' received no response: Server failure
Possible reasons why DNSSEC validation may be unavailable:
  - The local /etc/resolv.conf file specifies a DNS resolver that does not
      validate DNSSEC signatures (that's $queue_directory/etc/resolv.conf when a
      Postfix daemon runs in a chroot jail).
- The local system library does not pass on the "DNSSEC validated"
      bit to Postfix, or Postfix does not know how to ask the library to do
      that.
    
 
By default, the DNSSEC probe asks for the DNS root zone NS
    records, because resolvers should always have that information cached. If
    Postfix runs on a network where the DNS root zone is not reachable, specify
    a different probe, or specify an empty dnssec_probe value to disable the
    feature.
This feature is available in Postfix 3.6 and later. It was
    backported to Postfix versions 3.5.9, 3.4.19, 3.3.16. 3.2.21.
Don't remove queue files and save them to the "saved" mail queue. This
  is a debugging aid. To inspect the envelope information and content of a
  Postfix queue file, use the postcat(1) command.
The sender address of postmaster notifications that are generated by the mail
  system. All mail to this address is silently discarded, in order to terminate
  mail bounce loops.
The maximal number of addresses remembered by the address duplicate filter for
  aliases(5) or virtual(5) alias expansion, or for showq(8)
  queue displays.
The sender_dependent_default_transport_maps search string that will be used
  instead of the null sender address.
This feature is available in Postfix 2.7 and later.
The lookup key to be used in local_login_sender_maps tables, instead of the null
  sender address.
This feature is available in Postfix 3.6 and later.
The recipient of mail addressed to the null address. Postfix does not accept
  such addresses in SMTP commands, but they may still be created locally as the
  result of configuration or software error.
The sender_dependent_relayhost_maps search string that will be used instead of
  the null sender address.
This feature is available in Postfix 2.5 and later. With earlier
    versions, sender_dependent_relayhost_maps lookups were skipped for the null
    sender address.
Report mail delivery errors to the address specified with the non-standard
  Errors-To: message header, instead of the envelope sender address (this
  feature is removed with Postfix version 2.2, is turned off by default with
  Postfix version 2.1, and is always turned on with older Postfix versions).
Enable 'transitional' compatibility between IDNA2003 and IDNA2008, when
  converting UTF-8 domain names to/from the ASCII form that is used for DNS
  lookups. Specify "yes" for compatibility with Postfix <= 3.1 (not
  recommended). This affects the conversion of domain names that contain for
  example the German sz and the Greek zeta. See
  http://unicode.org/cldr/utility/idna.jsp for more examples.
This feature is available in Postfix 3.2 and later.
Enable long, non-repeating, queue IDs (queue file names). The benefit of
  non-repeating names is simpler logfile analysis and easier queue migration
  (there is no need to run "postsuper" to change queue file names that
  don't match their message file inode number).
Note: see below for how to convert long queue file names to
    Postfix <= 2.8.
Changing the parameter value to "yes" has the following
    effects:
  - Existing queue file names are not affected.
- New queue files are created with names such as 3Pt2mN2VXxznjll. These are
      encoded in a 52-character alphabet that contains digits (0-9), upper-case
      letters (B-Z) and lower-case letters (b-z). For safety reasons the vowels
      (AEIOUaeiou) are excluded from the alphabet. The name format is: 6 or more
      characters for the time in seconds, 4 characters for the time in
      microseconds, the 'z'; the remainder is the file inode number encoded in
      the first 51 characters of the 52-character alphabet.
- New messages have a Message-ID header with
      queueID@myhostname.
- The mailq (postqueue -p) output has a wider Queue ID column. The number of
      whitespace-separated fields is not changed.
- The hash_queue_depth algorithm uses the first characters of the queue file
      creation time in microseconds, after conversion into hexadecimal
      representation. This produces the same queue hashing behavior as if the
      queue file name was created with "enable_long_queue_ids = no".
    
 
Changing the parameter value to "no" has the following
    effects:
  - Existing long queue file names are renamed to the short form (while
      running "postfix reload" or "postsuper").
- New queue files are created with names such as C3CD21F3E90 from a
      hexadecimal alphabet that contains digits (0-9) and upper-case letters
      (A-F). The name format is: 5 characters for the time in microseconds; the
      remainder is the file inode number.
- New messages have a Message-ID header with
      YYYYMMDDHHMMSS.queueid@myhostname, where
      YYYYMMDDHHMMSS are the year, month, day, hour, minute and
    second.
- The mailq (postqueue -p) output has the same format as with Postfix <=
      2.8.
- The hash_queue_depth algorithm uses the first characters of the queue file
      name, with the hexadecimal representation of the file creation time in
      microseconds.
    
 
Before migration to Postfix <= 2.8, the following commands are
    required to convert long queue file names into short names:
# postfix stop
# postconf enable_long_queue_ids=no
# postsuper
Repeat the postsuper command until it reports no more queue file
    name changes.
This feature is available in Postfix 2.9 and later.
Enable support for the original recipient address after an address is rewritten
  to a different address (for example with aliasing or with canonical mapping).
The original recipient address is used as follows:
  - Final delivery
- With "enable_original_recipient = yes", the original recipient
      address is stored in the X-Original-To message header. This header
      may be used to distinguish between different recipients that share the
      same mailbox.
    
 
- Recipient deduplication
- With "enable_original_recipient = yes", the cleanup(8)
      daemon performs duplicate recipient elimination based on the content of
      (original recipient, maybe-rewritten recipient) pairs. Otherwise, the
      cleanup(8) daemon performs duplicate recipient elimination based
      only on the maybe-rewritten recipient address.
    
 
Note: with Postfix <= 3.2 the "setting
    enable_original_recipient = no" breaks address verification for
    addresses that are aliased or otherwise rewritten (Postfix is unable to
    store the address verification result under the original probe destination
    address; instead, it can store the result only under the rewritten
  address).
This feature is available in Postfix 2.1 and later. Postfix
    version 2.0 behaves as if this parameter is always set to yes.
    Postfix versions before 2.0 have no support for the original recipient
    address.
Enable non-delivery, success, and delay notifications that link to the original
  message by including a References: and In-Reply-To: header with the original
  Message-ID value. There are advantages and disadvantages to consider.
  -  advantage 
- This allows mail readers to present a delivery status notification in the
      same email thread as the original message.
    
 
-  disadvantage 
- This makes it easy for users to mistakenly delete the whole email thread
      (all related messages), instead of deleting only the non-delivery
      notification.
    
 
This feature is available in Postfix 3.6 and later.
The recipient of postmaster notifications about mail delivery problems that are
  caused by policy, resource, software or protocol errors. These notifications
  are enabled with the notify_classes parameter.
The name of the error(8) pseudo delivery agent. This service always
  returns mail as undeliverable.
This feature is available in Postfix 2.0 and later.
Restrict the characters that the local(8) delivery agent allows in $name
  expansions of $command_execution_directory. Characters outside the allowed set
  are replaced by underscores.
This feature is available in Postfix 2.2 and later.
When delivering to an alias "aliasname" that has an
  "owner-aliasname" companion alias, set the envelope sender
  address to the expansion of the "owner-aliasname" alias.
  Normally, Postfix sets the envelope sender address to the name of the
  "owner-aliasname" alias.
The list of environment variables that a Postfix process will export to
  non-Postfix processes. The TZ variable is needed for sane time keeping on
  System-V-ish systems.
Specify a list of names and/or name=value pairs, separated by
    whitespace or comma. Specify "{ name=value }" to protect
    whitespace or comma in parameter values (whitespace after the opening
    "{" and before the closing "}" is ignored). The form
    name=value is supported with Postfix version 2.1 and later; the use of {} is
    supported with Postfix 3.0 and later.
Example:
export_environment = TZ PATH=/bin:/usr/bin
The maximal number of recipient addresses that Postfix will extract from message
  headers when mail is submitted with "sendmail -t".
This feature was removed in Postfix version 2.1.
Optional list of relay hosts for SMTP destinations that can't be found or that
  are unreachable. With Postfix 2.3 this parameter is renamed to
  smtp_fallback_relay.
By default, mail is returned to the sender when a destination is
    not found, and delivery is deferred when a destination is unreachable.
The fallback relays must be SMTP destinations. Specify a domain,
    host, host:port, [host]:port, [address] or [address]:port; the form [host]
    turns off MX lookups. If you specify multiple SMTP destinations, Postfix
    will try them in the specified order.
Note: before Postfix 2.2, do not use the fallback_relay feature
    when relaying mail for a backup or primary MX domain. Mail would loop
    between the Postfix MX host and the fallback_relay host when the final
    destination is unavailable.
  - In main.cf specify "relay_transport = relay",
- In master.cf specify "-o fallback_relay =" (i.e., empty) at the
      end of the relay entry.
- In transport maps, specify "relay:nexthop..." as the
      right-hand side for backup or primary MX domain entries.
    
 
Postfix version 2.2 and later will not use the fallback_relay
    feature for destinations that it is MX host for.
Optional message delivery transport that the local(8) delivery agent
  should use for names that are not found in the aliases(5) or UNIX
  password database.
The precedence of local(8) delivery features from high to
    low is: aliases, .forward files, mailbox_transport_maps, mailbox_transport,
    mailbox_command_maps, mailbox_command, home_mailbox, mail_spool_directory,
    fallback_transport_maps, fallback_transport and luser_relay.
Optional lookup tables with per-recipient message delivery transports for
  recipients that the local(8) delivery agent could not find in the
  aliases(5) or UNIX password database.
The precedence of local(8) delivery features from high to
    low is: aliases, .forward files, mailbox_transport_maps, mailbox_transport,
    mailbox_command_maps, mailbox_command, home_mailbox, mail_spool_directory,
    fallback_transport_maps, fallback_transport and luser_relay.
For safety reasons, this feature does not allow $number
    substitutions in regular expression maps.
This feature is available in Postfix 2.3 and later.
Optional list of destinations that are eligible for per-destination logfiles
  with mail that is queued to those destinations.
By default, Postfix maintains "fast flush" logfiles only
    for destinations that the Postfix SMTP server is willing to relay to (i.e.
    the default is: "fast_flush_domains = $relay_domains"; see the
    relay_domains parameter in the postconf(5) manual).
Specify a list of hosts or domains, "/file/name"
    patterns or "type:table" lookup tables, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when the domain or its parent
    domain appears as lookup key.
Pattern matching of domain names is controlled by the presence or
    absence of "fast_flush_domains" in the
    parent_domain_matches_subdomains parameter value.
Specify "fast_flush_domains =" (i.e., empty) to disable
    the feature altogether.
The time after which an empty per-destination "fast flush" logfile is
  deleted.
You can specify the time as a number, or as a number followed by a
    letter that indicates the time unit: s=seconds, m=minutes, h=hours, d=days,
    w=weeks. The default time unit is days.
The time after which a non-empty but unread per-destination "fast
  flush" logfile needs to be refreshed. The contents of a logfile are
  refreshed by requesting delivery of all messages listed in the logfile.
You can specify the time as a number, or as a number followed by a
    letter that indicates the time unit: s=seconds, m=minutes, h=hours, d=days,
    w=weeks. The default time unit is hours.
Force specific internal tests to fail, to test the handling of errors that are
  difficult to reproduce otherwise.
The name of the flush(8) service. This service maintains per-destination
  logfiles with the queue file names of mail that is queued for those
  destinations.
This feature is available in Postfix 2.0 and later.
The maximal number of attempts to fork() a child process.
The delay between attempts to fork() a child process.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
Restrict the characters that the local(8) delivery agent allows in $name
  expansions of $forward_path. Characters outside the allowed set are replaced
  by underscores.
The local(8) delivery agent search list for finding a .forward file with
  user-specified delivery methods. The first file that is found is used.
The forward_path value is not subject to Postfix configuration
    parameter $name expansion. Instead, the following $name expansions are done
    on forward_path before the search actually happens. The result of $name
    expansion is filtered with the character set that is specified with the
    forward_expansion_filter parameter.
  - $user
- The recipient's username.
    
 
- $shell
- The recipient's login shell pathname.
    
 
- $home
- The recipient's home directory.
    
 
- $recipient
- The full recipient address.
    
 
- $extension
- The optional recipient address extension.
    
 
- $domain
- The recipient domain.
    
 
- $local
- The entire recipient localpart.
    
 
- $recipient_delimiter
- The address extension delimiter that was found in the recipient address
      (Postfix 2.11 and later), or the 'first' delimiter specified with the
      system-wide recipient address extension delimiter (Postfix 3.5.22, 3.5.12,
      3.7.8, 3.8.3 and later). Historically, this was always the system-wide
      recipient address extension delimiter (Postfix 2.10 and earlier).
    
 
- ${name?value}
- ${name?{value}} (Postfix >= 3.0)
- Expands to value when $name is non-empty.
    
 
- ${name:value}
- ${name:{value}} (Postfix >= 3.0)
- Expands to value when $name is empty.
    
 
- ${name?{value1}:{value2}} (Postfix >= 3.0)
- Expands to value1 when $name is non-empty, value2
      otherwise.
    
 
Instead of $name you can also specify ${name} or $(name).
Examples:
forward_path = /var/forward/$user
forward_path =
    /var/forward/$user/.forward$recipient_delimiter$extension,
    /var/forward/$user/.forward
Update the local(8) delivery agent's idea of the Delivered-To: address
  (see prepend_delivered_header) only once, at the start of a delivery attempt;
  do not update the Delivered-To: address while expanding aliases or .forward
  files.
This feature is available in Postfix 2.3 and later. With older
    Postfix releases, the behavior is as if this parameter is set to
    "no". The old setting can be expensive with deeply nested aliases
    or .forward files. When an alias or .forward file changes the Delivered-To:
    address, it ties up one queue file and one cleanup process instance while
    mail is being forwarded.
The number of subdirectory levels for queue directories listed with the
  hash_queue_names parameter. Queue hashing is implemented by creating one or
  more levels of directories with one-character names. Originally, these
  directory names were equal to the first characters of the queue file name,
  with the hexadecimal representation of the file creation time in microseconds.
With long queue file names, queue hashing produces the same
    results as with short names. The file creation time in microseconds is
    converted into hexadecimal form before the result is used for queue hashing.
    The base 16 encoding gives finer control over the number of subdirectories
    than is possible with the base 52 encoding of long queue file names.
After changing the hash_queue_names or hash_queue_depth parameter,
    execute the command "postfix reload".
The names of queue directories that are split across multiple subdirectory
  levels.
Before Postfix version 2.2, the default list of hashed queues was
    significantly larger. Claims about improvements in file system technology
    suggest that hashing of the incoming and active queues is no longer needed.
    Fewer hashed directories speed up the time needed to restart Postfix.
After changing the hash_queue_names or hash_queue_depth parameter,
    execute the command "postfix reload".
The maximal number of address tokens are allowed in an address message header.
  Information that exceeds the limit is discarded. The limit is enforced by the
  cleanup(8) server.
Optional lookup tables for content inspection of primary non-MIME message
  headers, as specified in the header_checks(5) manual page.
The format of the Postfix-generated From: header. This setting affects
  the appearance of 'full name' information when a local program such as
  /bin/mail submits a message without a From: header through the Postfix
  sendmail(1) command.
Specify one of the following:
  - standard (default)
- Produce a header formatted as "From: name
      <address>". This is the default as of Postfix
      3.3.
    
 
- obsolete
- Produce a header formatted as "From: address
      (name)". This is the behavior prior to Postfix
      3.3.
    
 
Notes:
  - Postfix generates the format "From: address" when
      name information is unavailable or the envelope sender address is
      empty. This is the same behavior as prior to Postfix 3.3.
- In the standard form, the name will be quoted if it contains
      specials as defined in RFC 5322, or the "!%" address
      operators.
- The Postfix sendmail(1) command gets name information from
      the -F command-line option, from the NAME environment
      variable, or from the UNIX password file.
    
 
This feature is available in Postfix 3.3 and later.
The maximal amount of memory in bytes for storing a message header. If a header
  is larger, the excess is discarded. The limit is enforced by the
  cleanup(8) server.
Log warnings about problematic configuration settings, and provide helpful
  suggestions.
This feature is available in Postfix 2.0 and later.
Optional pathname of a mailbox file relative to a local(8) user's home
  directory.
Specify a pathname ending in "/" for qmail-style
    delivery.
The precedence of local(8) delivery features from high to
    low is: aliases, .forward files, mailbox_transport_maps, mailbox_transport,
    mailbox_command_maps, mailbox_command, home_mailbox, mail_spool_directory,
    fallback_transport_maps, fallback_transport and luser_relay.
Examples:
home_mailbox = Mailbox
home_mailbox = Maildir/
The maximal number of Received: message headers that is allowed in the primary
  message headers. A message that exceeds the limit is bounced, in order to stop
  a mailer loop.
The location of Postfix HTML files that describe how to build, configure or
  operate a specific Postfix subsystem or feature.
Ignore DNS MX lookups that produce no response. By default, the Postfix SMTP
  client defers delivery and tries again after some delay. This behavior is
  required by the SMTP standard.
Specify "ignore_mx_lookup_error = yes" to force a DNS A
    record lookup instead. This violates the SMTP standard and can result in
    mis-delivery of mail.
When SRV record lookup fails, fall back to MX or IP address lookup as if SRV
  record lookup was not enabled.
This feature is available in Postfix 3.8 and later.
The list of environment variables that a privileged Postfix process will import
  from a non-Postfix parent process, or name=value environment overrides.
  Unprivileged utilities will enforce the name=value overrides, but otherwise
  will not change their process environment. Examples of relevant environment
  variables:
  - TZ
- May be needed for sane time keeping on most System-V-ish systems.
    
 
- DISPLAY
- Needed for debugging Postfix daemons with an X-windows debugger.
    
 
- XAUTHORITY
- Needed for debugging Postfix daemons with an X-windows debugger.
    
 
- MAIL_CONFIG
- Needed to make "postfix -c" work.
    
 
- POSTLOG_SERVICE
- Needed to make "maillog_file" work during daemon process
      initialization.
    
 
- POSTLOG_HOSTNAME
- Needed to make "maillog_file" work during daemon process
      initialization.
    
 
Specify a list of names and/or name=value pairs, separated by
    whitespace or comma. Specify "{ name=value }" to protect
    whitespace or comma in environment variable values (whitespace after the
    opening "{" and before the closing "}" is ignored). The
    form name=value is supported with Postfix version 2.1 and later; the use of
    {} is supported with Postfix 3.0 and later.
Time to pause before accepting a new message, when the message arrival rate
  exceeds the message delivery rate. This feature is turned on by default (it's
  disabled on SCO UNIX due to an SCO bug).
With the default 100 Postfix SMTP server process limit,
    "in_flow_delay = 1s" limits the mail inflow to 100 messages per
    second above the number of messages delivered per second.
Specify 0 to disable the feature. Valid delays are 0..10.
The local network interface addresses that this mail system receives mail on.
  Specify "all" to receive mail on all network interfaces (default),
  and "loopback-only" to receive mail on loopback network interfaces
  only (Postfix version 2.2 and later). The parameter also controls delivery of
  mail to user@[ip.address].
Note 1: you need to stop and start Postfix when this parameter
    changes.
Note 2: address information may be enclosed inside [], but this
    form is not required here.
When inet_interfaces specifies just one IPv4 and/or IPv6 address
    that is not a loopback address, the Postfix SMTP client will use this
    address as the IP source address for outbound mail. Support for IPv6 is
    available in Postfix version 2.2 and later.
On a multi-homed firewall with separate Postfix instances
    listening on the "inside" and "outside" interfaces, this
    can prevent each instance from being able to reach remote SMTP servers on
    the "other side" of the firewall. Setting smtp_bind_address to
    0.0.0.0 avoids the potential problem for IPv4, and setting
    smtp_bind_address6 to :: solves the problem for IPv6.
A better solution for multi-homed firewalls is to leave
    inet_interfaces at the default value and instead use explicit IP addresses
    in the master.cf SMTP server definitions. This preserves the Postfix SMTP
    client's loop detection, by ensuring that each side of the firewall knows
    that the other IP address is still the same host. Setting $inet_interfaces
    to a single IPv4 and/or IPV6 address is primarily useful with virtual
    hosting of domains on secondary IP addresses, when each IP address serves a
    different domain (and has a different $myhostname setting).
See also the proxy_interfaces parameter, for network addresses
    that are forwarded to Postfix by way of a proxy or address translator.
Examples:
inet_interfaces = all (DEFAULT)
inet_interfaces = loopback-only (Postfix version 2.2 and later)
inet_interfaces = 127.0.0.1
inet_interfaces = 127.0.0.1, [::1] (Postfix version 2.2 and later)
inet_interfaces = 192.168.1.2, 127.0.0.1
The Internet protocols Postfix will attempt to use when making or accepting
  connections. Specify one or more of "ipv4" or "ipv6",
  separated by whitespace or commas. The form "all" is equivalent to
  "ipv4, ipv6" or "ipv4", depending on whether the operating
  system implements IPv6.
With Postfix 2.8 and earlier the default is "ipv4". For
    backwards compatibility with these releases, the Postfix 2.9 and later
    upgrade procedure appends an explicit "inet_protocols = ipv4"
    setting to main.cf when no explicit setting is present. This compatibility
    workaround will be phased out as IPv6 deployment becomes more common.
This feature is available in Postfix 2.2 and later.
Note: you MUST stop and start Postfix after changing this
    parameter.
On systems that pre-date IPV6_V6ONLY support (RFC 3493), an IPv6
    server will also accept IPv4 connections, even when IPv4 is turned off with
    the inet_protocols parameter. On systems with IPV6_V6ONLY support, Postfix
    will use separate server sockets for IPv6 and IPv4, and each will accept
    only connections for the corresponding protocol.
When IPv4 support is enabled via the inet_protocols parameter,
    Postfix will look up DNS type A records, and will convert IPv4-in-IPv6
    client IP addresses (::ffff:1.2.3.4) to their original IPv4 form (1.2.3.4).
    The latter is needed on hosts that pre-date IPV6_V6ONLY support (RFC
  3493).
When IPv6 support is enabled via the inet_protocols parameter,
    Postfix will do DNS type AAAA record lookups.
When both IPv4 and IPv6 support are enabled, the Postfix SMTP
    client will choose the protocol as specified with the
    smtp_address_preference parameter. Postfix versions before 2.8 attempt to
    connect via IPv6 before attempting to use IPv4.
Examples:
inet_protocols = ipv4
inet_protocols = all (DEFAULT)
inet_protocols = ipv6
inet_protocols = ipv4, ipv6
The email address form that will be used in non-debug logging (info, warning,
  etc.). As of Postfix 3.5 when an address localpart contains spaces or other
  special characters, the localpart will be quoted, for example:
  
    from=<"name with spaces"@example.com>
Older Postfix versions would log the internal (unquoted) form:
  
    from=<name with spaces@example.com>
The external and internal forms are identical for the vast
    majority of email addresses that contain no spaces or other special
    characters in the localpart.
The logging in external form is consistent with the address form
    that Postfix 3.2 and later prefer for most table lookups. This is therefore
    the more useful form for non-debug logging.
Specify "info_log_address_format = internal" for
    backwards compatibility.
Postfix uses the unquoted form internally, because an attacker can
    specify an email address in different forms by playing games with quotes and
    backslashes. An attacker should not be able to use such games to circumvent
    Postfix access policies.
This feature is available in Postfix 3.5 and later.
The initial per-destination concurrency level for parallel delivery to the same
  destination. With per-destination recipient limit > 1, a destination is a
  domain, otherwise it is a recipient.
Use transport_initial_destination_concurrency to specify a
    transport-specific override, where transport is the master.cf name of
    the message delivery transport (Postfix 2.5 and later).
Warning: with concurrency of 1, one bad message can be enough to
    block all mail to a site.
What categories of Postfix-generated mail are subject to before-queue content
  inspection by non_smtpd_milters, header_checks and body_checks. Specify zero
  or more of the following, separated by whitespace or comma.
  - bounce
- Inspect the content of delivery status notifications.
    
 
- notify
- Inspect the content of postmaster notifications by the smtp(8) and
      smtpd(8) processes.
    
 
NOTE: It's generally not safe to enable content inspection of
    Postfix-generated email messages. The user is warned.
This feature is available in Postfix 2.3 and later.
The numerical Postfix SMTP server response code when the client HELO or EHLO
  command parameter is rejected by the reject_invalid_helo_hostname restriction.
Do not change this unless you have a complete understanding of RFC
    5321.
The time after which a client closes an idle internal communication channel. The
  purpose is to allow Postfix daemon processes to terminate voluntarily after
  they become idle. This is used, for example, by the Postfix address resolving
  and rewriting clients.
With Postfix 2.4 the default value was reduced from 100s to
  5s.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The time limit for sending or receiving information over an internal
  communication channel. The purpose is to break out of deadlock situations. If
  the time limit is exceeded the software aborts with a fatal error.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The time after which a client closes an active internal communication channel.
  The purpose is to allow Postfix daemon processes to terminate voluntarily
  after reaching their client limit. This is used, for example, by the Postfix
  address resolving and rewriting clients.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
Optional setting that avoids lookups in the services(5) database. This
  feature was implemented to address inconsistencies in the name of the port
  "465" service. The ABNF is:
  
  known_tcp_ports = empty | name-to-port *("," name-to-port)
  
  name-to-port = 1*(service-name "=') port-number
  
The comma is required. Whitespace is optional but it cannot appear
    inside a service name or port number.
This feature is available in Postfix 3.6 and later.
Upon input, long lines are chopped up into pieces of at most this length; upon
  delivery, long lines are reconstructed.
The initial OpenLDAP LMDB database size limit in bytes. Each time a database
  becomes full, its size limit is doubled.
This feature is available in Postfix 2.11 and later.
The LMTP-specific version of the smtp_address_preference configuration
  parameter. See there for details.
This feature is available in Postfix 2.8 and later.
The LMTP-specific version of the smtp_address_verify_target configuration
  parameter. See there for details.
This feature is available in Postfix 3.0 and later.
When a remote LMTP server announces no DSN support, assume that the server
  performs final delivery, and send "delivered" delivery status
  notifications instead of "relayed". The default setting is backwards
  compatible to avoid the infinitesimal possibility of breaking existing
  LMTP-based content filters.
The LMTP-specific version of the smtp_balance_inet_protocols configuration
  parameter. See there for details.
This feature is available in Postfix 3.3 and later.
The LMTP-specific version of the smtp_bind_address configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_bind_address6 configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_bind_address_enforce configuration
  parameter. See there for details.
This feature is available in Postfix 3.7 and later.
The LMTP-specific version of the smtp_body_checks configuration parameter. See
  there for details.
This feature is available in Postfix 2.5 and later.
Keep Postfix LMTP client connections open for up to $max_idle seconds. When the
  LMTP client receives a request for the same connection the connection is
  reused.
This parameter is available in Postfix version 2.2 and earlier.
    With Postfix version 2.3 and later, see lmtp_connection_cache_on_demand,
    lmtp_connection_cache_destinations, or lmtp_connection_reuse_time_limit.
The effectiveness of cached connections will be determined by the
    number of remote LMTP servers in use, and the concurrency limit specified
    for the Postfix LMTP client. Cached connections are closed under any of the
    following conditions:
  - The Postfix LMTP client idle time limit is reached. This limit is
      specified with the Postfix max_idle configuration parameter.
- A delivery request specifies a different destination than the one
      currently cached.
- The per-process limit on the number of delivery requests is reached. This
      limit is specified with the Postfix max_use configuration parameter.
- Upon the onset of another delivery request, the remote LMTP server
      associated with the current session does not respond to the RSET command.
    
 
Most of these limitations have been with the Postfix connection
    cache that is shared among multiple LMTP client programs.
The LMTP-specific version of the smtp_cname_overrides_servername configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The Postfix LMTP client time limit for completing a TCP connection, or zero (use
  the operating system built-in time limit). When no connection can be made
  within the deadline, the LMTP client tries the next address on the mail
  exchanger list.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
Example:
lmtp_connect_timeout = 30s
The LMTP-specific version of the smtp_connection_cache_destinations
  configuration parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_connection_cache_on_demand configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_connection_cache_time_limit configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_connection_reuse_count_limit configuration
  parameter. See there for details.
This feature is available in Postfix 2.11 and later.
The LMTP-specific version of the smtp_connection_reuse_time_limit configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The Postfix LMTP client time limit for sending the LMTP ".", and for
  receiving the remote LMTP server response. When no response is received within
  the deadline, a warning is logged that the mail may be delivered multiple
  times.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The Postfix LMTP client time limit for sending the LMTP DATA command, and for
  receiving the remote LMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The Postfix LMTP client time limit for sending the LMTP message content. When
  the connection stalls for more than $lmtp_data_xfer_timeout the LMTP client
  terminates the transfer.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The LMTP-specific version of the smtp_defer_if_no_mx_address_found configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_delivery_status_filter configuration
  parameter. See there for details.
This feature is available in Postfix 3.0 and later.
The maximal number of parallel deliveries to the same destination via the lmtp
  message delivery transport. This limit is enforced by the queue manager. The
  message delivery transport name is the first field in the entry in the
  master.cf file.
The maximal number of recipients per message for the lmtp message delivery
  transport. This limit is enforced by the queue manager. The message delivery
  transport name is the first field in the entry in the master.cf file.
Setting this parameter to a value of 1 changes the meaning of
    lmtp_destination_concurrency_limit from concurrency per domain into
    concurrency per recipient.
Lookup tables, indexed by the remote LMTP server address, with case insensitive
  lists of LHLO keywords (pipelining, starttls, auth, etc.) that the Postfix
  LMTP client will ignore in the LHLO response from a remote LMTP server. See
  lmtp_discard_lhlo_keywords for details. The table is not indexed by hostname
  for consistency with smtpd_discard_ehlo_keyword_address_maps.
This feature is available in Postfix 2.3 and later.
A case insensitive list of LHLO keywords (pipelining, starttls, auth, etc.) that
  the Postfix LMTP client will ignore in the LHLO response from a remote LMTP
  server.
This feature is available in Postfix 2.3 and later.
Notes:
  - Specify the silent-discard pseudo keyword to prevent this action
      from being logged.
- Use the lmtp_discard_lhlo_keyword_address_maps feature to discard LHLO
      keywords selectively.
    
 
Optional filter for Postfix LMTP client DNS lookup results. See
  smtp_dns_reply_filter for details including an example.This feature is available in Postfix 3.0 and later.
The LMTP-specific version of the smtp_dns_resolver_options configuration
  parameter. See there for details.
This feature is available in Postfix 2.8 and later.
The LMTP-specific version of the smtp_dns_support_level configuration parameter.
  See there for details.
This feature is available in Postfix 2.11 and later.
The LMTP-specific version of the smtp_enforce_tls configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
Optional list of relay hosts for LMTP destinations that can't be found or that
  are unreachable. In main.cf elements are separated by whitespace or commas.
By default, mail is returned to the sender when a destination is
    not found, and delivery is deferred when a destination is unreachable.
The fallback relays must be TCP destinations, specified without a
    leading "inet:" prefix. Specify a host or host:port. Since MX
    lookups do not apply with LMTP, there is no need to use the
    "[host]" or "[host]:port" forms. If you specify multiple
    LMTP destinations, Postfix will try them in the specified order.
This feature is available in Postfix 3.1 and later.
The LMTP-specific version of the smtp_generic_maps configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_header_checks configuration parameter. See
  there for details.
This feature is available in Postfix 2.5 and later.
The LMTP-specific version of the smtp_host_lookup configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The hostname to send in the LMTP LHLO command.
The default value is the machine hostname. Specify a hostname or
    [ip.add.re.ss] or [ip:v6:add:re::ss].
This information can be specified in the main.cf file for all LMTP
    clients, or it can be specified in the master.cf file for a specific client,
    for example:
  
/etc/postfix/master.cf:
    mylmtp ... lmtp -o lmtp_lhlo_name=foo.bar.com
This feature is available in Postfix 2.3 and later.
The Postfix LMTP client time limit for sending the LHLO command, and for
  receiving the initial remote LMTP server response.
Time units: s (seconds), m (minutes), h (hours), d (days), w
    (weeks). The default time unit is s (seconds).
The LMTP-specific version of the smtp_line_length_limit configuration parameter.
  See there for details.
This feature is available in Postfix 2.3 and later.
The Postfix LMTP client time limit for sending the MAIL FROM command, and for
  receiving the remote LMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The LMTP-specific version of the smtp_mime_header_checks configuration
  parameter. See there for details.
This feature is available in Postfix 2.5 and later.
The LMTP-specific version of the smtp_min_data_rate configuration parameter. See
  there for details.
This feature is available in Postfix 3.7 and later.
The LMTP-specific version of the smtp_mx_address_limit configuration parameter.
  See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_mx_session_limit configuration parameter.
  See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_nested_header_checks configuration
  parameter. See there for details.
This feature is available in Postfix 2.5 and later.
The LMTP-specific version of the smtp_per_record_deadline configuration
  parameter. See there for details.
This feature is available in Postfix 2.9 and later.
The LMTP-specific version of the smtp_per_request_deadline configuration
  parameter. See there for details.
This feature is available in Postfix 3.7 and later.
The LMTP-specific version of the smtp_pix_workaround_delay_time configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_pix_workaround_maps configuration
  parameter. See there for details.
This feature is available in Postfix 2.4 and later.
The LMTP-specific version of the smtp_pix_workaround_threshold_time
  configuration parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_pix_workaround configuration parameter.
  See there for details.
This feature is available in Postfix 2.4 and later.
The Postfix LMTP client time limit for sending the QUIT command, and for
  receiving the remote LMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The LMTP-specific version of the smtp_quote_rfc821_envelope configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_randomize_addresses configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The Postfix LMTP client time limit for sending the RCPT TO command, and for
  receiving the remote LMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The LMTP-specific version of the smtp_reply_filter configuration parameter. See
  there for details.
This feature is available in Postfix 2.7 and later.
The Postfix LMTP client time limit for sending the RSET command, and for
  receiving the remote LMTP server response. The LMTP client sends RSET in order
  to finish a recipient address probe, or to verify that a cached connection is
  still alive.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The LMTP-specific version of the smtp_sasl_auth_cache_name configuration
  parameter. See there for details.
This feature is available in Postfix 2.5 and later.
The LMTP-specific version of the smtp_sasl_auth_cache_time configuration
  parameter. See there for details.
This feature is available in Postfix 2.5 and later.
Enable SASL authentication in the Postfix LMTP client.
The LMTP-specific version of the smtp_sasl_auth_soft_bounce configuration
  parameter. See there for details.
This feature is available in Postfix 2.5 and later.
The LMTP-specific version of the smtp_sasl_mechanism_filter configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
Optional Postfix LMTP client lookup tables with one username:password entry per
  host or domain. If a remote host or domain has no username:password entry,
  then the Postfix LMTP client will not attempt to authenticate to the remote
  host.
Implementation-specific information that is passed through to the SASL plug-in
  implementation that is selected with lmtp_sasl_type. Typically this
  specifies the name of a configuration file or rendezvous point.
This feature is available in Postfix 2.3 and later.
SASL security options; as of Postfix 2.3 the list of available features depends
  on the SASL client implementation that is selected with lmtp_sasl_type.
The following security features are defined for the cyrus
    client SASL implementation:
  - noplaintext
- Disallow authentication methods that use plaintext passwords.
    
 
- noactive
- Disallow authentication methods that are vulnerable to non-dictionary
      active attacks.
    
 
- nodictionary
- Disallow authentication methods that are vulnerable to passive dictionary
      attacks.
    
 
- noanonymous
- Disallow anonymous logins.
    
 
Example:
lmtp_sasl_security_options = noplaintext
The LMTP-specific version of the smtp_sasl_tls_security_options configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_sasl_tls_verified_security_options
  configuration parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The SASL plug-in type that the Postfix LMTP client should use for
  authentication. The available types are listed with the "postconf
  -A" command.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_send_dummy_mail_auth configuration
  parameter. See there for details.
This feature is available in Postfix 2.9 and later.
Send an XFORWARD command to the remote LMTP server when the LMTP LHLO server
  response announces XFORWARD support. This allows an lmtp(8) delivery
  agent, used for content filter message injection, to forward the name,
  address, protocol and HELO name of the original client to the content filter
  and downstream LMTP server. Before you change the value to yes, it is best to
  make sure that your content filter supports this command.
This feature is available in Postfix 2.1 and later.
The LMTP-specific version of the smtp_sender_dependent_authentication
  configuration parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_skip_5xx_greeting configuration parameter.
  See there for details.
This feature is available in Postfix 2.3 and later.
Wait for the response to the LMTP QUIT command.
The LMTP-specific version of the smtp_starttls_timeout configuration parameter.
  See there for details.
This feature is available in Postfix 2.3 and later.
The default TCP port that the Postfix LMTP client connects to. Specify a
  symbolic name (see services(5)) or a numeric port.
The LMTP-specific version of the smtp_tls_CAfile configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_CApath configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_block_early_mail_reply configuration
  parameter. See there for details.
This feature is available in Postfix 2.7 and later.
The LMTP-specific version of the smtp_tls_cert_file configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_chain_files configuration parameter.
  See there for details.
This feature is available in Postfix 3.4 and later.
The LMTP-specific version of the smtp_tls_ciphers configuration parameter. See
  there for details.
This feature is available in Postfix 2.6 and later.
The LMTP-specific version of the smtp_tls_connection_reuse configuration
  parameter. See there for details.
This feature is available in Postfix 3.4 and later.
The LMTP-specific version of the smtp_tls_dcert_file configuration parameter.
  See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_dkey_file configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_eccert_file configuration parameter.
  See there for details.
This feature is available in Postfix 2.6 and later, when Postfix
    is compiled and linked with OpenSSL 1.0.0 or later.
The LMTP-specific version of the smtp_tls_eckey_file configuration parameter.
  See there for details.
This feature is available in Postfix 2.6 and later, when Postfix
    is compiled and linked with OpenSSL 1.0.0 or later.
The LMTP-specific version of the smtp_tls_enforce_peername configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_exclude_ciphers configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_fingerprint_cert_match configuration
  parameter. See there for details.
This feature is available in Postfix 2.5 and later.
The LMTP-specific version of the smtp_tls_fingerprint_digest configuration
  parameter. See there for details.
This feature is available in Postfix 2.5 and later.
The LMTP-specific version of the smtp_tls_force_insecure_host_tlsa_lookup
  configuration parameter. See there for details.
This feature is available in Postfix 2.11 and later.
The LMTP-specific version of the smtp_tls_key_file configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_loglevel configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_mandatory_ciphers configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_mandatory_exclude_ciphers
  configuration parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_mandatory_protocols configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_note_starttls_offer configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_per_site configuration parameter. See
  there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_policy_maps configuration parameter.
  See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_protocols configuration parameter. See
  there for details.
This feature is available in Postfix 2.6 and later.
The LMTP-specific version of the smtp_tls_scert_verifydepth configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_secure_cert_match configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_security_level configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_servername configuration parameter.
  See there for details.
This feature is available in Postfix 3.4 and later.
The LMTP-specific version of the smtp_tls_session_cache_database configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_session_cache_timeout configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_trust_anchor_file configuration
  parameter. See there for details.
This feature is available in Postfix 2.11 and later.
The LMTP-specific version of the smtp_tls_verify_cert_match configuration
  parameter. See there for details.
This feature is available in Postfix 2.3 and later.
The LMTP-specific version of the smtp_tls_wrappermode configuration parameter.
  See there for details.
This feature is available in Postfix 3.0 and later.
The LMTP-specific version of the smtp_use_tls configuration parameter. See there
  for details.
This feature is available in Postfix 2.3 and later.
The Postfix LMTP client time limit for sending the XFORWARD command, and for
  receiving the remote LMTP server response.
In case of problems the client does NOT try the next address on
    the mail exchanger list.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
Optional shell program for local(8) delivery to non-Postfix commands. By
  default, non-Postfix commands are executed directly; commands are given to the
  default shell (typically, /bin/sh) only when they contain shell meta
  characters or shell built-in commands.
"sendmail's restricted shell" (smrsh) is what most
    people will use in order to restrict what programs can be run from e.g.
    .forward files (smrsh is part of the Sendmail distribution).
Note: when a shell program is specified, it is invoked even when
    the command contains no shell built-in commands or meta characters.
Example:
local_command_shell = /some/where/smrsh -c
local_command_shell = /bin/bash -c
Optional filter for the local(8) delivery agent to change the status code
  or explanatory text of successful or unsuccessful deliveries. See
  default_delivery_status_filter for details.
This feature is available in Postfix 3.0 and later.
The maximal number of parallel deliveries via the local mail delivery transport
  to the same recipient (when "local_destination_recipient_limit = 1")
  or the maximal number of parallel deliveries to the same local domain (when
  "local_destination_recipient_limit > 1"). This limit is enforced
  by the queue manager. The message delivery transport name is the first field
  in the entry in the master.cf file.
A low limit of 2 is recommended, just in case someone has an
    expensive shell command in a .forward file or in an alias (e.g., a mailing
    list manager). You don't want to run lots of those at the same time.
The maximal number of recipients per message delivery via the local mail
  delivery transport. This limit is enforced by the queue manager. The message
  delivery transport name is the first field in the entry in the master.cf file.
Setting this parameter to a value > 1 changes the meaning of
    local_destination_concurrency_limit from concurrency per recipient into
    concurrency per domain.
Rewrite or add message headers in mail from these clients, updating incomplete
  addresses with the domain name in $myorigin or $mydomain, and adding missing
  headers.
See the append_at_myorigin and append_dot_mydomain parameters for
    details of how domain names are appended to incomplete addresses.
See remote_header_rewrite_domain to optionally rewrite or add
    message headers in mail from other clients.
Specify a list of zero or more of the following:
  - permit_inet_interfaces
- Append the domain name in $myorigin or $mydomain when the client IP
      address matches $inet_interfaces. This is enabled by default.
    
 
- permit_mynetworks
- Append the domain name in $myorigin or $mydomain when the client IP
      address matches any network or network address listed in $mynetworks. This
      setting will not prevent remote mail header address rewriting when mail
      from a remote client is forwarded by a neighboring system.
    
 
- permit_sasl_authenticated 
- Append the domain name in $myorigin or $mydomain when the client is
      successfully authenticated via the RFC 4954 (AUTH) protocol.
    
 
- permit_tls_clientcerts 
- Append the domain name in $myorigin or $mydomain when the remote SMTP
      client TLS certificate fingerprint or public key fingerprint (Postfix 2.9
      and later) is listed in $relay_clientcerts. The fingerprint digest
      algorithm is configurable via the smtpd_tls_fingerprint_digest parameter
      (hard-coded as md5 prior to Postfix version 2.5).
    
 The default algorithm is sha256 with Postfix >= 3.6 and the
      compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
      the default algorithm is md5. The best-practice algorithm is now
      sha256. Recent advances in hash function cryptanalysis have led to
      md5 and sha1 being deprecated in favor of sha256. However, as long as
      there are no known "second pre-image" attacks against the older
      algorithms, their use in this context, though not recommended, is still
      likely safe.
 
- permit_tls_all_clientcerts 
- Append the domain name in $myorigin or $mydomain when the remote SMTP
      client TLS certificate is successfully verified, regardless of whether it
      is listed on the server, and regardless of the certifying authority.
    
 
- check_address_map type:table 
- type:table 
- Append the domain name in $myorigin or $mydomain when the client IP
      address matches the specified lookup table. The lookup result is ignored,
      and no subnet lookup is done. This is suitable for, e.g., pop-before-smtp
      lookup tables.
    
 
Examples:
The Postfix < 2.2 backwards compatible setting: always rewrite
    message headers, and always append my own domain to incomplete header
    addresses.
  
local_header_rewrite_clients = static:all
The purist (and default) setting: rewrite headers only in mail
    from Postfix sendmail and in SMTP mail from this machine.
  
local_header_rewrite_clients = permit_inet_interfaces
The intermediate setting: rewrite header addresses and append
    $myorigin or $mydomain information only with mail from Postfix sendmail,
    from local clients, or from authorized SMTP clients.
Note: this setting will not prevent remote mail header address
    rewriting when mail from a remote client is forwarded by a neighboring
    system.
  
local_header_rewrite_clients = permit_mynetworks,
    permit_sasl_authenticated permit_tls_clientcerts
    check_address_map hash:/etc/postfix/pop-before-smtp
A list of lookup tables that are searched by the UNIX login name, and that
  return a list of allowed envelope sender patterns separated by space or comma.
  These sender patterns are enforced by the Postfix postdrop(1) command.
  The default is backwards-compatible: every user may specify any sender
  envelope address.
When no UNIX login name is available, the postdrop(1)
    command will prepend "uid:" to the numerical UID and use
    that instead.
This feature ignores address extensions in the user-specified
    envelope sender address.
The following sender patterns are special; these cannot be used as
    part of a longer pattern.
  -  * 
- This pattern allows any envelope sender address.
    
 
-  <> 
- This pattern allows the empty envelope sender address. See the
      empty_address_local_login_sender_maps_lookup_key configuration parameter.
    
 
-  @domain
- This pattern allows an envelope sender address when the '@' and
      domain part match.
    
 
Examples:
/etc/postfix/main.cf:
    # Allow root and postfix full control, anyone else can only
    # send mail as themselves. Use "uid:" followed by the numerical
    # UID when the UID has no entry in the UNIX password file.
    local_login_sender_maps =
        inline:{ { root = * }, { postfix = * } },
        pcre:/etc/postfix/login_senders
/etc/postfix/login_senders:
   # Allow both the bare username and the user@domain forms.
    /(.+)/ $1 $1@example.com
This feature is available in Postfix 3.6 and later.
Lookup tables with all names or addresses of local recipients: a recipient
  address is local when its domain matches $mydestination, $inet_interfaces or
  $proxy_interfaces. Specify @domain as a wild-card for domains that do not have
  a valid recipient list. Technically, tables listed with $local_recipient_maps
  are used as lists: Postfix needs to know only if a lookup string is found or
  not, but it does not use the result from table lookup.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
If this parameter is non-empty (the default), then the Postfix
    SMTP server will reject mail for unknown local users.
To turn off local recipient checking in the Postfix SMTP server,
    specify "local_recipient_maps =" (i.e. empty).
The default setting assumes that you use the default Postfix local
    delivery agent for local delivery. You need to update the
    local_recipient_maps setting if:
  - You redefine the local delivery agent in master.cf.
- You redefine the "local_transport" setting in main.cf.
- You use the "luser_relay", "mailbox_transport", or
      "fallback_transport" feature of the Postfix local(8)
      delivery agent.
    
 
Details are described in the LOCAL_RECIPIENT_README file.
Beware: if the Postfix SMTP server runs chrooted, you need to
    access the passwd file via the proxymap(8) service, in order to
    overcome chroot access restrictions. The alternative, maintaining a copy of
    the system password file in the chroot jail is not practical.
Examples:
local_recipient_maps =
The default mail delivery transport and next-hop destination for final delivery
  to domains listed with mydestination, and for [ipaddress] destinations that
  match $inet_interfaces or $proxy_interfaces. This information can be overruled
  with the transport(5) table.
By default, local mail is delivered to the transport called
    "local", which is just the name of a service that is defined the
    master.cf file.
Specify a string of the form transport:nexthop, where
    transport is the name of a mail delivery transport defined in
    master.cf. The :nexthop destination is optional; its syntax is
    documented in the manual page of the corresponding delivery agent.
Beware: if you override the default local delivery agent then you
    need to review the LOCAL_RECIPIENT_README document, otherwise the SMTP
    server may reject mail for local recipients.
Optional catch-all destination for unknown local(8) recipients. By
  default, mail for unknown recipients in domains that match $mydestination,
  $inet_interfaces or $proxy_interfaces is returned as undeliverable.
The luser_relay value is not subject to Postfix configuration
    parameter $name expansion. Instead, the following $name expansions are
  done:
  - $domain
- The recipient domain.
    
 
- $extension
- The recipient address extension.
    
 
- $home
- The recipient's home directory.
    
 
- $local
- The entire recipient address localpart.
    
 
- $recipient
- The full recipient address.
    
 
- $recipient_delimiter
- The address extension delimiter that was found in the recipient address
      (Postfix 2.11 and later), or the system-wide recipient address extension
      delimiter (Postfix 2.10 and earlier).
    
 
- $shell
- The recipient's login shell.
    
 
- $user
- The recipient username.
    
 
- ${name?value}
- ${name?{value}} (Postfix >= 3.0)
- Expands to value when $name is non-empty.
    
 
- ${name:value}
- ${name:{value}} (Postfix >= 3.0)
- Expands to value when $name is empty.
    
 
- ${name?{value1}:{value2}} (Postfix >= 3.0)
- Expands to value1 when $name is non-empty, value2
      otherwise.
    
 
Instead of $name you can also specify ${name} or $(name).
Note: luser_relay works only for the Postfix local(8)
    delivery agent.
Note: if you use this feature for accounts not in the UNIX
    password file, then you must specify "local_recipient_maps ="
    (i.e. empty) in the main.cf file, otherwise the Postfix SMTP server will
    reject mail for non-UNIX accounts with "User unknown in local recipient
    table".
Examples:
luser_relay = $user@other.host
luser_relay = $local@other.host
luser_relay = admin+$local
The mail system name that is displayed in Received: headers, in the SMTP
  greeting banner, and in bounced mail.
The UNIX system account that owns the Postfix queue and most Postfix daemon
  processes. Specify the name of an unprivileged user account that does not
  share a user or group ID with other accounts, and that owns no other files or
  processes on the system. In particular, don't specify nobody or daemon. PLEASE
  USE A DEDICATED USER ID AND GROUP ID.
When this parameter value is changed you need to re-run
    "postfix set-permissions" (with Postfix version 2.0
    and earlier: "/etc/postfix/post-install
  set-permissions".
The Postfix release date, in "YYYYMMDD" format.
The directory where local(8) UNIX-style mailboxes are kept. The default
  setting depends on the system type. Specify a name ending in / for
  maildir-style delivery.
Note: maildir delivery is done with the privileges of the
    recipient. If you use the mail_spool_directory setting for maildir style
    delivery, then you must create the top-level maildir directory in advance.
    Postfix will not create it.
Examples:
mail_spool_directory = /var/mail
mail_spool_directory = /var/spool/mail
The version of the mail system. Stable releases are named
  major.minor.patchlevel. Experimental releases also
  include the release date. The version string can be used in, for example, the
  SMTP greeting banner.
Optional external command that the local(8) delivery agent should use for
  mailbox delivery. The command is run with the user ID and the primary group ID
  privileges of the recipient. Exception: command delivery for root executes
  with $default_privs privileges. This is not a problem, because 1) mail for
  root should always be aliased to a real user and 2) don't log in as root, use
  "su" instead.
The following environment variables are exported to the
  command:
  - CLIENT_ADDRESS
- Remote client network address. Available in Postfix version 2.2 and later.
    
 
- CLIENT_HELO
- Remote client EHLO command parameter. Available in Postfix version 2.2 and
      later.
    
 
- CLIENT_HOSTNAME
- Remote client hostname. Available in Postfix version 2.2 and later.
    
 
- CLIENT_PROTOCOL
- Remote client protocol. Available in Postfix version 2.2 and later.
    
 
- DOMAIN
- The domain part of the recipient address.
    
 
- EXTENSION
- The optional address extension.
    
 
- HOME
- The recipient home directory.
    
 
- LOCAL
- The recipient address localpart.
    
 
- LOGNAME
- The recipient's username.
    
 
- ORIGINAL_RECIPIENT
- The entire recipient address, before any address rewriting or aliasing.
    
 
- RECIPIENT
- The full recipient address.
    
 
- SASL_METHOD
- SASL authentication method specified in the remote client AUTH command.
      Available in Postfix version 2.2 and later.
    
 
- SASL_SENDER
- SASL sender address specified in the remote client MAIL FROM command.
      Available in Postfix version 2.2 and later.
    
 
- SASL_USER
- SASL username specified in the remote client AUTH command. Available in
      Postfix version 2.2 and later.
    
 
- SENDER
- The full sender address.
    
 
- SHELL
- The recipient's login shell.
    
 
- USER
- The recipient username.
    
 
Unlike other Postfix configuration parameters, the mailbox_command
    parameter is not subjected to $name substitutions. This is to make it easier
    to specify shell syntax (see example below).
If you can, avoid shell meta characters because they will force
    Postfix to run an expensive shell process. If you're delivering via
    "procmail" then running a shell won't make a noticeable difference
    in the total cost.
Note: if you use the mailbox_command feature to deliver mail
    system-wide, you must set up an alias that forwards mail for root to a real
    user.
The precedence of local(8) delivery features from high to
    low is: aliases, .forward files, mailbox_transport_maps, mailbox_transport,
    mailbox_command_maps, mailbox_command, home_mailbox, mail_spool_directory,
    fallback_transport_maps, fallback_transport and luser_relay.
Examples:
mailbox_command = /some/where/procmail
mailbox_command = /some/where/procmail -a "$EXTENSION"
mailbox_command = /some/where/maildrop -d "$USER"
        -f "$SENDER" "$EXTENSION"
Optional lookup tables with per-recipient external commands to use for
  local(8) mailbox delivery. Behavior is as with mailbox_command.
The precedence of local(8) delivery features from high to
    low is: aliases, .forward files, mailbox_transport_maps, mailbox_transport,
    mailbox_command_maps, mailbox_command, home_mailbox, mail_spool_directory,
    fallback_transport_maps, fallback_transport and luser_relay.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
How to lock a UNIX-style local(8) mailbox before attempting delivery. For
  a list of available file locking methods, use the "postconf
  -l" command.
This setting is ignored with maildir style delivery,
    because such deliveries are safe without explicit locks.
Note: The dotlock method requires that the recipient UID or
    GID has write access to the parent directory of the mailbox file.
Note: the default setting of this parameter is system
  dependent.
The maximal size of any local(8) individual mailbox or maildir file, or
  zero (no limit). In fact, this limits the size of any file that is written to
  upon local delivery, including files written by external commands that are
  executed by the local(8) delivery agent. The value cannot exceed
  LONG_MAX (typically, a 32-bit or 64-bit signed integer).
This limit must not be smaller than the message size limit.
Optional message delivery transport that the local(8) delivery agent
  should use for mailbox delivery to all local recipients, whether or not they
  are found in the UNIX passwd database.
The precedence of local(8) delivery features from high to
    low is: aliases, .forward files, mailbox_transport_maps, mailbox_transport,
    mailbox_command_maps, mailbox_command, home_mailbox, mail_spool_directory,
    fallback_transport_maps, fallback_transport and luser_relay.
Optional lookup tables with per-recipient message delivery transports to use for
  local(8) mailbox delivery, whether or not the recipients are found in
  the UNIX passwd database.
The precedence of local(8) delivery features from high to
    low is: aliases, .forward files, mailbox_transport_maps, mailbox_transport,
    mailbox_command_maps, mailbox_command, home_mailbox, mail_spool_directory,
    fallback_transport_maps, fallback_transport and luser_relay.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
For safety reasons, this feature does not allow $number
    substitutions in regular expression maps.
This feature is available in Postfix 2.3 and later.
The name of an optional logfile that is written by the Postfix
  postlogd(8) service. An empty value selects logging to
  syslogd(8). Specify "/dev/stdout" to select logging to
  standard output. Stdout logging requires that Postfix is started with
  "postfix start-fg".
Note 1: The maillog_file parameter value must contain a prefix
    that is specified with the maillog_file_prefixes parameter.
Note 2: Some Postfix non-daemon programs may still log information
    to syslogd(8), before they have processed their configuration
    parameters and command-line options.
This feature is available in Postfix 3.4 and later.
The program to run after rotating $maillog_file with "postfix
  logrotate". The command is run with the rotated logfile name as its first
  argument.
This feature is available in Postfix 3.4 and later.
A list of allowed prefixes for a maillog_file value. This is a safety feature to
  contain the damage from a single configuration mistake. Specify one or more
  prefix strings, separated by comma or whitespace.
This feature is available in Postfix 3.4 and later.
The format of the suffix to append to $maillog_file while rotating the file with
  "postfix logrotate". See strftime(3) for syntax. The default
  suffix, YYYYMMDD-HHMMSS, allows logs to be rotated frequently.
This feature is available in Postfix 3.4 and later.
Sendmail compatibility feature that specifies where the Postfix mailq(1)
  command is installed. This command can be used to list the Postfix mail queue.
Where the Postfix manual pages are installed.
Obsolete feature: use the reject_rbl_client feature instead.
The numerical Postfix SMTP server response code when a remote SMTP client
  request is blocked by the reject_rbl_client, reject_rhsbl_client,
  reject_rhsbl_reverse_client, reject_rhsbl_sender or reject_rhsbl_recipient
  restriction.
Do not change this unless you have a complete understanding of RFC
    5321.
What addresses are subject to address masquerading.
By default, address masquerading is limited to envelope sender
    addresses, and to header sender and header recipient addresses. This allows
    you to use address masquerading on a mail gateway while still being able to
    forward mail to users on individual machines.
Specify zero or more of: envelope_sender, envelope_recipient,
    header_sender, header_recipient
Optional list of domains whose subdomain structure will be stripped off in email
  addresses.
The list is processed left to right, and processing stops at the
    first match. Thus,
  
masquerade_domains = foo.example.com example.com
strips "user@any.thing.foo.example.com" to
    "user@foo.example.com", but strips
    "user@any.thing.else.example.com" to
  "user@example.com".
A domain name prefixed with ! means do not masquerade this domain
    or its subdomains. Thus,
  
masquerade_domains = !foo.example.com example.com
does not change "user@any.thing.foo.example.com" or
    "user@foo.example.com", but strips
    "user@any.thing.else.example.com" to
  "user@example.com".
Note: with Postfix version 2.2, message header address
    masquerading happens only when message header address rewriting is
  enabled:
  - The message is received with the Postfix sendmail(1) command,
- The message is received from a network client that matches
      $local_header_rewrite_clients,
- The message is received from the network, and the
      remote_header_rewrite_domain parameter specifies a non-empty value.
    
 
To get the behavior before Postfix version 2.2, specify
    "local_header_rewrite_clients = static:all".
Example:
masquerade_domains = $mydomain
Optional list of user names that are not subjected to address masquerading, even
  when their addresses match $masquerade_domains.
By default, address masquerading makes no exceptions.
Specify a list of user names, "/file/name" or
    "type:table" patterns, separated by commas and/or whitespace. The
    list is matched left to right, and the search stops on the first match. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a name matches a lookup
    key (the lookup result is ignored). Continue long lines by starting the next
    line with whitespace. Specify "!pattern" to exclude a name from
    the list. The form "!/file/name" is supported only in Postfix
    version 2.4 and later.
Examples:
masquerade_exceptions = root, mailer-daemon
masquerade_exceptions = root
Selectively disable master(8) listener ports by service type or by
  service name and type. Specify a list of service types ("inet",
  "unix", "fifo", or "pass") or
  "name/type" tuples, where "name" is the first field of a
  master.cf entry and "type" is a service type. As with other Postfix
  matchlists, a search stops at the first match. Specify "!pattern" to
  exclude a service from the list. By default, all master(8) listener
  ports are enabled.
Note: this feature does not support "/file/name" or
    "type:table" patterns, nor does it support wildcards such as
    "*" or "all". This is intentional.
Examples:
# With Postfix 2.6..2.10 use '.' instead of '/'.
# Turn on all master(8) listener ports (the default).
master_service_disable =
# Turn off only the main SMTP listener port.
master_service_disable = smtp/inet
# Turn off all TCP/IP listener ports.
master_service_disable = inet
# Turn off all TCP/IP listener ports except "foo".
master_service_disable = !foo/inet, inet
This feature is available in Postfix 2.6 and later.
The maximum amount of time that an idle Postfix daemon process waits for an
  incoming connection before terminating voluntarily. This parameter is ignored
  by the Postfix queue manager and by other long-lived Postfix daemon processes.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The maximal number of incoming connections that a Postfix daemon process will
  service before terminating voluntarily. This parameter is ignored by the
  Postfix queue manager and by other long-lived Postfix daemon processes.
The maximal time between attempts to deliver a deferred message.
This parameter should be set to a value greater than or equal to
    $minimal_backoff_time. See also $queue_run_delay.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
Consider a message as undeliverable, when delivery fails with a temporary error,
  and the time in the queue has reached the maximal_queue_lifetime limit.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is d (days).
Specify 0 when mail delivery should be tried only once.
Names of message headers that the cleanup(8) daemon will remove after
  applying header_checks(5) and before invoking Milter applications. The
  default setting is compatible with Postfix < 3.0.
Specify a list of header names, separated by comma or space. Names
    are matched in a case-insensitive manner. The list of supported header names
    is limited only by available memory.
This feature is available in Postfix 3.0 and later.
The set of characters that Postfix will reject in message content. The usual
  C-like escape sequences are recognized: \a \b \f \n \r \t \v \ddd (up
  to three octal digits) and \\.
Note 1: this feature does not recognize text that requires MIME
    decoding. It inspects raw message content, just like header_checks and
    body_checks.
Note 2: this feature is disabled with
    "receive_override_options = no_header_body_checks".
Example:
message_reject_characters = \0
This feature is available in Postfix 2.3 and later.
The maximal size in bytes of a message, including envelope information. The
  value cannot exceed LONG_MAX (typically, a 32-bit or 64-bit signed integer).
Note: be careful when making changes. Excessively small values
    will result in the loss of non-delivery notifications, when a bounce message
    size exceeds the local or remote MTA's message size limit.
The set of characters that Postfix will remove from message content. The usual
  C-like escape sequences are recognized: \a \b \f \n \r \t \v \ddd (up
  to three octal digits) and \\.
Note 1: this feature does not recognize text that requires MIME
    decoding. It inspects raw message content, just like header_checks and
    body_checks.
Note 2: this feature is disabled with
    "receive_override_options = no_header_body_checks".
Example:
message_strip_characters = \0
This feature is available in Postfix 2.3 and later.
The location of non-executable files that are shared among multiple Postfix
  instances, such as postfix-files, dynamicmaps.cf, and the multi-instance
  template files main.cf.proto and master.cf.proto. This directory should
  contain only Postfix-related files. Typically, the meta_directory parameter
  has the same default as the config_directory parameter (/etc/postfix or
  /usr/local/etc/postfix).
For backwards compatibility with Postfix versions 2.6..2.11,
    specify "meta_directory = $daemon_directory" in main.cf before
    installing or upgrading Postfix, or specify "meta_directory =
    /path/name" on the "make makefiles", "make install"
    or "make upgrade" command line.
This feature is available in Postfix 3.0 and later.
The time limit for sending an SMTP command to a Milter (mail filter)
  application, and for receiving the response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.3 and later.
The macros that are sent to Milter (mail filter) applications after completion
  of an SMTP connection. See MILTER_README for a list of available macro names
  and their meanings.
This feature is available in Postfix 2.3 and later.
The time limit for connecting to a Milter (mail filter) application, and for
  negotiating protocol options.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.3 and later.
The time limit for sending message content to a Milter (mail filter)
  application, and for receiving the response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.3 and later.
The macros that are sent to version 4 or higher Milter (mail filter)
  applications after the SMTP DATA command. See MILTER_README for a list of
  available macro names and their meanings.
This feature is available in Postfix 2.3 and later.
The default action when a Milter (mail filter) response is unavailable (for
  example, bad Postfix configuration or Milter failure). Specify one of the
  following:
  - accept
- Proceed as if the mail filter was not present.
    
 
- reject
- Reject all further commands in this session with a permanent status code.
    
 
- tempfail
- Reject all further commands in this session with a temporary status code.
    
 
- quarantine
- Like "accept", but freeze the message in the "hold"
      queue. Available with Postfix 2.6 and later.
    
 
This feature is available in Postfix 2.3 and later.
The macros that are sent to Milter (mail filter) applications after the message
  end-of-data. See MILTER_README for a list of available macro names and their
  meanings.
This feature is available in Postfix 2.3 and later.
The macros that are sent to Milter (mail filter) applications after the end of
  the message header. See MILTER_README for a list of available macro names and
  their meanings.
This feature is available in Postfix 2.5 and later.
Optional lookup tables for content inspection of message headers that are
  produced by Milter applications. See the header_checks(5) manual page
  available actions. Currently, PREPEND is not implemented.
The following example sends all mail that is marked as SPAM to a
    spam handling machine. Note that matches are case-insensitive by
  default.
/etc/postfix/main.cf:
    milter_header_checks = pcre:/etc/postfix/milter_header_checks
/etc/postfix/milter_header_checks:
    /^X-SPAM-FLAG:\s+YES/ FILTER mysmtp:sanitizer.example.com:25
The milter_header_checks mechanism could also be used for
    allowlisting. For example it could be used to skip heavy content inspection
    for DKIM-signed mail from known friendly domains.
This feature is available in Postfix 2.7, and as an optional patch
    for Postfix 2.6.
The macros that are sent to Milter (mail filter) applications after the SMTP
  HELO or EHLO command. See MILTER_README for a list of available macro names
  and their meanings.
This feature is available in Postfix 2.3 and later.
The {daemon_name} macro value for Milter (mail filter) applications. See
  MILTER_README for a list of available macro names and their meanings.
This feature is available in Postfix 2.3 and later.
Optional list of name=value pairs that specify default values for
  arbitrary macros that Postfix may send to Milter applications. These defaults
  are used when there is no corresponding information from the message delivery
  context.
Specify name=value or {name=value} pairs separated
    by comma or whitespace. Enclose a pair in "{}" when a value
    contains comma or whitespace (this form ignores whitespace after the
    enclosing "{", around the "=", and before the enclosing
    "}").
This feature is available in Postfix 3.1 and later.
The {v} macro value for Milter (mail filter) applications. See MILTER_README for
  a list of available macro names and their meanings.
This feature is available in Postfix 2.3 and later.
The macros that are sent to Milter (mail filter) applications after the SMTP
  MAIL FROM command. See MILTER_README for a list of available macro names and
  their meanings.
This feature is available in Postfix 2.3 and later.
The mail filter protocol version and optional protocol extensions for
  communication with a Milter application; prior to Postfix 2.6 the default
  protocol is 2. Postfix sends this version number during the initial protocol
  handshake. It should match the version number that is expected by the mail
  filter application (or by its Milter library).
Protocol versions:
  - 2
- Use Sendmail 8 mail filter protocol version 2 (default with Sendmail
      version 8.11 .. 8.13 and Postfix version 2.3 .. 2.5).
    
 
- 3
- Use Sendmail 8 mail filter protocol version 3.
    
 
- 4
- Use Sendmail 8 mail filter protocol version 4.
    
 
- 6
- Use Sendmail 8 mail filter protocol version 6 (default with Sendmail
      version 8.14 and Postfix version 2.6).
    
 
Protocol extensions:
  - no_header_reply
- Specify this when the Milter application will not reply for each
      individual message header.
    
 
This feature is available in Postfix 2.3 and later.
The macros that are sent to Milter (mail filter) applications after the SMTP
  RCPT TO command. See MILTER_README for a list of available macro names and
  their meanings.
This feature is available in Postfix 2.3 and later.
The macros that are sent to version 3 or higher Milter (mail filter)
  applications after an unknown SMTP command. See MILTER_README for a list of
  available macro names and their meanings.
This feature is available in Postfix 2.3 and later.
The maximal length of MIME multipart boundary strings. The MIME processor is
  unable to distinguish between boundary strings that do not differ in the first
  $mime_boundary_length_limit characters.
This feature is available in Postfix 2.0 and later.
Optional lookup tables for content inspection of MIME related message headers,
  as described in the header_checks(5) manual page.
This feature is available in Postfix 2.0 and later.
The maximal recursion level that the MIME processor will handle. Postfix refuses
  mail that is nested deeper than the specified limit.
This feature is available in Postfix 2.0 and later.
The minimal time between attempts to deliver a deferred message; prior to
  Postfix 2.4 the default value was 1000s.
This parameter also limits the time an unreachable destination is
    kept in the short-term, in-memory, destination status cache.
This parameter should be set greater than or equal to
    $queue_run_delay. See also $maximal_backoff_time.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
An optional list of non-default Postfix configuration directories; these
  directories belong to additional Postfix instances that share the Postfix
  executable files and documentation with the default Postfix instance, and that
  are started, stopped, etc., together with the default Postfix instance.
  Specify a list of pathnames separated by comma or whitespace.
When $multi_instance_directories is empty, the postfix(1)
    command runs in single-instance mode and operates on a single Postfix
    instance only. Otherwise, the postfix(1) command runs in
    multi-instance mode and invokes the multi-instance manager specified with
    the multi_instance_wrapper parameter. The multi-instance manager in turn
    executes postfix(1) commands for the default instance and for all
    Postfix instances in $multi_instance_directories.
Currently, this parameter setting is ignored except for the
    default main.cf file.
This feature is available in Postfix 2.6 and later.
Allow this Postfix instance to be started, stopped, etc., by a multi-instance
  manager. By default, new instances are created in a safe state that prevents
  them from being started inadvertently. This parameter is reserved for the
  multi-instance manager.
This feature is available in Postfix 2.6 and later.
The optional instance group name of this Postfix instance. A group identifies
  closely-related Postfix instances that the multi-instance manager can start,
  stop, etc., as a unit. This parameter is reserved for the multi-instance
  manager.
This feature is available in Postfix 2.6 and later.
The optional instance name of this Postfix instance. This name becomes also the
  default value for the syslog_name parameter.
This feature is available in Postfix 2.6 and later.
The pathname of a multi-instance manager command that the postfix(1)
  command invokes when the multi_instance_directories parameter value is
  non-empty. The pathname may be followed by initial command arguments separated
  by whitespace; shell metacharacters such as quotes are not supported in this
  context.
The postfix(1) command invokes the manager command with the
    postfix(1) non-option command arguments on the manager command line,
    and with all installation configuration parameters exported into the manager
    command process environment. The manager command in turn invokes the
    postfix(1) command for individual Postfix instances as "postfix
    -c config_directory command".
This feature is available in Postfix 2.6 and later.
The numerical Postfix SMTP server response code when a remote SMTP client
  request is blocked by the reject_multi_recipient_bounce restriction.
Do not change this unless you have a complete understanding of RFC
    5321.
This feature is available in Postfix 2.1 and later.
The list of domains that are delivered via the $local_transport mail delivery
  transport. By default this is the Postfix local(8) delivery agent which
  looks up all recipients in /etc/passwd and /etc/aliases. The SMTP server
  validates recipient addresses with $local_recipient_maps and rejects
  non-existent recipients. See also the local domain class in the
  ADDRESS_CLASS_README file.
The default mydestination value specifies names for the local
    machine only. On a mail domain gateway, you should also include
  $mydomain.
The $local_transport delivery method is also selected for mail
    addressed to user@[the.net.work.address] of the mail system (the IP
    addresses specified with the inet_interfaces and proxy_interfaces
    parameters).
Warnings:
  - Do not specify the names of virtual domains - those domains are specified
      elsewhere. See VIRTUAL_README for more information.
- Do not specify the names of domains that this machine is backup MX host
      for. See STANDARD_CONFIGURATION_README for how to set up backup MX
    hosts.
- By default, the Postfix SMTP server rejects mail for recipients not listed
      with the local_recipient_maps parameter. See the postconf(5) manual
      for a description of the local_recipient_maps and
      unknown_local_recipient_reject_code parameters.
    
 
Specify a list of host or domain names, "/file/name" or
    "type:table" patterns, separated by commas and/or whitespace. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a name matches a lookup
    key (the lookup result is ignored). Continue long lines by starting the next
    line with whitespace.
Examples:
mydestination = $myhostname, localhost.$mydomain $mydomain
mydestination = $myhostname, localhost.$mydomain www.$mydomain, ftp.$mydomain
The internet domain name of this mail system. The default is to use $myhostname
  minus the first component, or "localdomain" (Postfix 2.3 and later).
  $mydomain is used as a default value for many other configuration parameters.
Example:
mydomain = domain.tld
The internet hostname of this mail system. The default is to use the
  fully-qualified domain name (FQDN) from gethostname(), or to use the non-FQDN
  result from gethostname() and append ".$mydomain". $myhostname is
  used as a default value for many other configuration parameters.
Example:
myhostname = host.example.com
The list of "trusted" remote SMTP clients that have more privileges
  than "strangers".
In particular, "trusted" SMTP clients are allowed to
    relay mail through Postfix. See the smtpd_relay_restrictions parameter
    description in the postconf(5) manual.
You can specify the list of "trusted" network addresses
    by hand or you can let Postfix do it for you (which is the default). See the
    description of the mynetworks_style parameter for more information.
If you specify the mynetworks list by hand, Postfix ignores the
    mynetworks_style setting.
Specify a list of network addresses or network/netmask patterns,
    separated by commas and/or whitespace. Continue long lines by starting the
    next line with whitespace.
The netmask specifies the number of bits in the network part of a
    host address. You can also specify "/file/name" or
    "type:table" patterns. A "/file/name" pattern is
    replaced by its contents; a "type:table" lookup table is matched
    when a table entry matches a lookup string (the lookup result is
  ignored).
The list is matched left to right, and the search stops on the
    first match. Specify "!pattern" to exclude an address or network
    block from the list. The form "!/file/name" is supported only in
    Postfix version 2.4 and later.
Note 1: Pattern matching of domain names is controlled by the
    presence or absence of "mynetworks" in the
    parent_domain_matches_subdomains parameter value.
Note 2: IP version 6 address information must be specified inside
    [] in the mynetworks value, and in files specified with
    "/file/name". IP version 6 addresses contain the ":"
    character, and would otherwise be confused with a "type:table"
    pattern.
Note 3: CIDR ranges cannot be specified in hash tables. Use cidr
    tables if CIDR ranges are used.
Examples:
mynetworks = 127.0.0.0/8 168.100.189.0/28
mynetworks = !192.168.0.1, 192.168.0.0/28
mynetworks = 127.0.0.0/8 168.100.189.0/28 [::1]/128 [2001:240:587::]/64
mynetworks = $config_directory/mynetworks
mynetworks = hash:/etc/postfix/network_table
mynetworks = cidr:/etc/postfix/network_table.cidr
The method to generate the default value for the mynetworks parameter. This is
  the list of trusted networks for relay access control etc.
  - Specify "mynetworks_style = host" when Postfix should
      "trust" only the local machine.
- Specify "mynetworks_style = subnet" when Postfix should
      "trust" remote SMTP clients in the same IP subnetworks as the
      local machine. On Linux, this works correctly only with interfaces
      specified with the "ifconfig" or "ip" command.
- Specify "mynetworks_style = class" when Postfix should
      "trust" remote SMTP clients in the same IP class A/B/C networks
      as the local machine. Caution: this may cause Postfix to "trust"
      your entire provider's network. Instead, specify an explicit mynetworks
      list by hand, as described with the mynetworks configuration parameter.
    
 
The domain name that locally-posted mail appears to come from, and that locally
  posted mail is delivered to. The default, $myhostname, is adequate for small
  sites. If you run a domain with multiple machines, you should (1) change this
  to $mydomain and (2) set up a domain-wide alias database that aliases each
  user to user@that.users.mailhost.Example:
myorigin = $mydomain
Optional lookup tables for content inspection of non-MIME message headers in
  attached messages, as described in the header_checks(5) manual page.
This feature is available in Postfix 2.0 and later.
Sendmail compatibility feature that specifies the location of the
  newaliases(1) command. This command can be used to rebuild the
  local(8) aliases(5) database.
The numerical Postfix SMTP server reply code when a client request is rejected
  by the reject_non_fqdn_helo_hostname, reject_non_fqdn_sender or
  reject_non_fqdn_recipient restriction.
A list of Milter (mail filter) applications for new mail that does not arrive
  via the Postfix smtpd(8) server. This includes local submission via the
  sendmail(1) command line, new mail that arrives via the Postfix
  qmqpd(8) server, and old mail that is re-injected into the queue with
  "postsuper -r". Specify space or comma as a separator. See the
  MILTER_README document for details.
This feature is available in Postfix 2.3 and later.
The list of error classes that are reported to the postmaster. These postmaster
  notifications do not replace user notifications. The default is to report only
  the most serious problems. The paranoid may wish to turn on the policy (UCE
  and mail relaying) and protocol error (broken mail software) reports.
NOTE: postmaster notifications may contain confidential
    information such as SASL passwords or message content. It is the system
    administrator's responsibility to treat such information with care.
The error classes are:
  - bounce (also implies 2bounce)
- Send the postmaster copies of the headers of bounced mail, and send
      transcripts of SMTP sessions when Postfix rejects mail. The notification
      is sent to the address specified with the bounce_notice_recipient
      configuration parameter (default: postmaster).
    
 
- 2bounce
- Send undeliverable bounced mail to the postmaster. The notification is
      sent to the address specified with the 2bounce_notice_recipient
      configuration parameter (default: postmaster).
    
 
- data
- Send the postmaster a transcript of the SMTP session with an error because
      a critical data file was unavailable. The notification is sent to the
      address specified with the error_notice_recipient configuration parameter
      (default: postmaster).
    
 This feature is available in Postfix 2.9 and later.
 
- delay
- Send the postmaster copies of the headers of delayed mail (see
      delay_warning_time). The notification is sent to the address specified
      with the delay_notice_recipient configuration parameter (default:
      postmaster).
    
 
- policy
- Send the postmaster a transcript of the SMTP session when a client request
      was rejected because of (UCE) policy. The notification is sent to the
      address specified with the error_notice_recipient configuration parameter
      (default: postmaster).
    
 
- protocol
- Send the postmaster a transcript of the SMTP session in case of client or
      server protocol errors. The notification is sent to the address specified
      with the error_notice_recipient configuration parameter (default:
      postmaster).
    
 
- resource
- Inform the postmaster of mail not delivered due to resource problems. The
      notification is sent to the address specified with the
      error_notice_recipient configuration parameter (default: postmaster).
    
 
- software
- Inform the postmaster of mail not delivered due to software problems. The
      notification is sent to the address specified with the
      error_notice_recipient configuration parameter (default: postmaster).
    
 
Examples:
notify_classes = bounce, delay, policy, protocol, resource, software
notify_classes = 2bounce, resource, software
The location of the OpenSSL command line program openssl(1). This is used
  by the "postfix tls" command to create private keys,
  certificate signing requests, self-signed certificates, and to compute public
  key digests for DANE TLSA records. In multi-instance environments, this
  parameter is always determined from the configuration of the default Postfix
  instance.
Example:
  
/etc/postfix/main.cf:
    # NetBSD pkgsrc:
    openssl_path = /usr/pkg/bin/openssl
    # Local build:
    openssl_path = /usr/local/bin/openssl
This feature is available in Postfix 3.1 and later.
Enable special treatment for owner-listname entries in the
  aliases(5) file, and don't split owner-listname and
  listname-request address localparts when the recipient_delimiter is set
  to "-". This feature is useful for mailing lists.
A list of Postfix features where the pattern "example.com" also
  matches subdomains of example.com, instead of requiring an explicit
  ".example.com" pattern. This is planned backwards compatibility:
  eventually, all Postfix features are expected to require explicit
  ".example.com" style patterns when you really want to match
  subdomains.
The following Postfix feature names are supported.
  - Postfix version 1.0 and later
- debug_peer_list, fast_flush_domains, mynetworks,
      permit_mx_backup_networks, relay_domains, transport_maps
    
 
- Postfix version 1.1 and later
- qmqpd_authorized_clients, smtpd_access_maps,
    
 
- Postfix version 2.8 and later
- postscreen_access_list
    
 
- Postfix version 3.0 and later
- smtpd_client_event_limit_exceptions
    
 
Restrict the use of the permit_mx_backup SMTP access feature to only domains
  whose primary MX hosts match the listed networks. The parameter value syntax
  is the same as with the mynetworks parameter; note, however, that the default
  value is empty.Pattern matching of domain names is controlled by the presence or
    absence of "permit_mx_backup_networks" in the
    parent_domain_matches_subdomains parameter value.
The name of the pickup(8) service. This service picks up local mail
  submissions from the Postfix maildrop queue.
This feature is available in Postfix 2.0 and later.
Optional filter for the pipe(8) delivery agent to change the delivery
  status code or explanatory text of successful or unsuccessful deliveries. See
  default_delivery_status_filter for details.
This feature is available in Postfix 3.0 and later.
The numerical Postfix SMTP server response code when a request is rejected by
  the reject_plaintext_session restriction.
This feature is available in Postfix 2.3 and later.
The name of the postlogd(8) service entry in master.cf. This service
  appends logfile records to the file specified with the maillog_file parameter.
This feature is available in Postfix 3.4 and later.
How much time a postlogd(8) process may take to process a request before
  it is terminated by a built-in watchdog timer. This is a safety mechanism that
  prevents postlogd(8) from becoming non-responsive due to a bug in
  Postfix itself or in system software. This limit cannot be set under 10s.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 3.4 and later.
The postfix(1) commands that the postmulti(1) instance manager
  treats as "control" commands, that operate on running instances. For
  these commands, disabled instances are skipped.
This feature is available in Postfix 2.6 and later.
The postfix(1) commands that the postmulti(1) instance manager
  treats as "start" commands. For these commands, disabled instances
  are "checked" rather than "started", and failure to
  "start" a member instance of an instance group will abort the
  start-up of later instances.
This feature is available in Postfix 2.6 and later.
The postfix(1) commands that the postmulti(1) instance manager
  treats as "stop" commands. For these commands, disabled instances
  are skipped, and enabled instances are processed in reverse order.
This feature is available in Postfix 2.6 and later.
Permanent allow/denylist for remote SMTP client IP addresses.
  postscreen(8) searches this list immediately after a remote SMTP client
  connects. Specify a comma- or whitespace-separated list of commands (in upper
  or lower case) or lookup tables. The search stops upon the first command that
  fires for the client IP address.
  -  permit_mynetworks 
- Allowlist the client and terminate the search if the client IP address
      matches $mynetworks. Do not subject the client to any before/after 220
      greeting tests. Pass the connection immediately to a Postfix SMTP server
      process.
    
 Pattern matching of domain names is controlled by the presence or absence of
      "postscreen_access_list" in the parent_domain_matches_subdomains
      parameter value.
 
-  type:table 
- Query the specified lookup table. Each table lookup result is an access
      list, except that access lists inside a table cannot specify type:table
      entries.
    
 To discourage the use of hash, btree, etc. tables, there is no support for
      substring matching like smtpd(8). Use CIDR tables instead.
 
-  permit 
- Allowlist the client and terminate the search. Do not subject the client
      to any before/after 220 greeting tests. Pass the connection immediately to
      a Postfix SMTP server process.
    
 
-  reject 
- Denylist the client and terminate the search. Subject the client to the
      action configured with the postscreen_denylist_action configuration
      parameter.
    
 
-  dunno 
- All postscreen(8) access lists implicitly have this command at the
      end.
    
 When  dunno  is executed inside a lookup table, return from the
      lookup table and evaluate the next command.
 When  dunno  is executed outside a lookup table, terminate the
      search, and subject the client to the configured before/after 220 greeting
      tests.
 
Example:
/etc/postfix/main.cf:
    postscreen_access_list = permit_mynetworks,
        cidr:/etc/postfix/postscreen_access.cidr
    # Postfix < 3.6 use postscreen_blacklist_action.
    postscreen_denylist_action = enforce
/etc/postfix/postscreen_access.cidr:
    # Rules are evaluated in the order as specified.
    # Denylist 192.168.* except 192.168.0.1.
    192.168.0.1         dunno
    192.168.0.0/16      reject
This feature is available in Postfix 2.8.
A list of local postscreen(8) server IP addresses where a non-allowlisted
  remote SMTP client can obtain postscreen(8)'s temporary allowlist
  status. This status is required before the client can talk to a Postfix SMTP
  server process. By default, a client can obtain postscreen(8)'s
  allowlist status on any local postscreen(8) server IP address.
When postscreen(8) listens on both primary and backup MX
    addresses, the postscreen_allowlist_interfaces parameter can be configured
    to give the temporary allowlist status only when a client connects to a
    primary MX address. Once a client is allowlisted it can talk to a Postfix
    SMTP server on any address. Thus, clients that connect only to backup MX
    addresses will never become allowlisted, and will never be allowed to talk
    to a Postfix SMTP server process.
Specify a list of network addresses or network/netmask patterns,
    separated by commas and/or whitespace. The netmask specifies the number of
    bits in the network part of a host address. Continue long lines by starting
    the next line with whitespace.
You can also specify "/file/name" or
    "type:table" patterns. A "/file/name" pattern is
    replaced by its contents; a "type:table" lookup table is matched
    when a table entry matches a lookup string (the lookup result is
  ignored).
The list is matched left to right, and the search stops on the
    first match. Specify "!pattern" to exclude an address or network
    block from the list.
Note: IP version 6 address information must be specified inside []
    in the postscreen_allowlist_interfaces value, and in files specified with
    "/file/name". IP version 6 addresses contain the ":"
    character, and would otherwise be confused with a "type:table"
    pattern.
Example:
/etc/postfix/main.cf:
    # Don't allowlist connections to the backup IP address.
    # Postfix < 3.6 use postscreen_whitelist_interfaces.
    postscreen_allowlist_interfaces = !168.100.189.8, static:all
This feature is available in Postfix 3.6 and later.
Available as postscreen_whitelist_interfaces in Postfix 2.9 -
  3.5.
The action that postscreen(8) takes when a remote SMTP client sends a
  bare newline character, that is, a newline not preceded by carriage return.
  Specify one of the following:
  - ignore
- Ignore the failure of this test. Allow other tests to complete. Do
      not repeat this test before the result from some other test
      expires. This option is useful for testing and collecting statistics
      without blocking mail permanently.
    
 
- enforce
- Allow other tests to complete. Reject attempts to deliver mail with a 550
      SMTP reply, and log the helo/sender/recipient information. Repeat this
      test the next time the client connects.
    
 
- drop
- Drop the connection immediately with a 521 SMTP reply. Repeat this test
      the next time the client connects.
    
 
This feature is available in Postfix 2.8.
Enable "bare newline" SMTP protocol tests in the postscreen(8)
  server. These tests are expensive: a remote SMTP client must disconnect after
  it passes the test, before it can talk to a real Postfix SMTP server.
This feature is available in Postfix 2.8.
The amount of time that postscreen(8) will use the result from a
  successful "bare newline" SMTP protocol test. During this time, the
  client IP address is excluded from this test. The default is long because a
  remote SMTP client must disconnect after it passes the test, before it can
  talk to a real Postfix SMTP server.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is d
    (days).
This feature is available in Postfix 2.8.
Renamed to postscreen_denylist_action in Postfix 3.6.
This feature is available in Postfix 2.8 - 3.5.
The amount of time between postscreen(8) cache cleanup runs. Cache
  cleanup increases the load on the cache database and should therefore not be
  run frequently. This feature requires that the cache database supports the
  "delete" and "sequence" operators. Specify a zero interval
  to disable cache cleanup.
After each cache cleanup run, the postscreen(8) daemon logs
    the number of entries that were retained and dropped. A cleanup run is
    logged as "partial" when the daemon terminates early after
    "postfix reload", "postfix stop",
    or no requests for $max_idle seconds.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is h (hours).
This feature is available in Postfix 2.8.
Persistent storage for the postscreen(8) server decisions.
To share a postscreen(8) cache between multiple
    postscreen(8) instances, use "postscreen_cache_map =
    proxy:btree:/path/to/file". This requires Postfix version 2.9 or later;
    earlier proxymap(8) implementations don't support cache cleanup. For
    an alternative approach see the memcache_table(5) manpage.
This feature is available in Postfix 2.8.
The amount of time that postscreen(8) will cache an expired temporary
  allowlist entry before it is removed. This prevents clients from being logged
  as "NEW" just because their cache entry expired an hour ago. It also
  prevents the cache from filling up with clients that passed some deep protocol
  test once and never came back.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is d
    (days).
This feature is available in Postfix 2.8.
How many simultaneous connections any remote SMTP client is allowed to have with
  the postscreen(8) daemon. By default, this limit is the same as with
  the Postfix SMTP server. Note that the triage process can take several
  seconds, with the time spent in postscreen_greet_wait delay, and with the time
  spent talking to the postscreen(8) built-in dummy SMTP protocol engine.
This feature is available in Postfix 2.8.
The limit on the total number of commands per SMTP session for
  postscreen(8)'s built-in SMTP protocol engine. This SMTP engine defers
  or rejects all attempts to deliver mail, therefore there is no need to enforce
  separate limits on the number of junk commands and error commands.
This feature is available in Postfix 2.8.
A mechanism to transform commands from remote SMTP clients. See
  smtpd_command_filter for further details.
This feature is available in Postfix 2.8 and later.
The time limit to read an entire command line with postscreen(8)'s
  built-in SMTP protocol engine.
This feature is available in Postfix 2.8.
The action that postscreen(8) takes when a remote SMTP client is
  permanently denylisted with the postscreen_access_list parameter. Specify one
  of the following:
  - ignore (default)
- Ignore this result. Allow other tests to complete. Repeat this test the
      next time the client connects. This option is useful for testing and
      collecting statistics without blocking mail.
    
 
- enforce
- Allow other tests to complete. Reject attempts to deliver mail with a 550
      SMTP reply, and log the helo/sender/recipient information. Repeat this
      test the next time the client connects.
    
 
- drop
- Drop the connection immediately with a 521 SMTP reply. Repeat this test
      the next time the client connects.
    
 
This feature is available in Postfix 3.6 and later.
Available as postscreen_blacklist_action in Postfix 2.8 - 3.5.
Disable the SMTP VRFY command in the postscreen(8) daemon. See
  disable_vrfy_command for details.
This feature is available in Postfix 2.8.
Lookup tables, indexed by the remote SMTP client address, with case insensitive
  lists of EHLO keywords (pipelining, starttls, auth, etc.) that the
  postscreen(8) server will not send in the EHLO response to a remote
  SMTP client. See smtpd_discard_ehlo_keywords for details. The table is not
  searched by hostname for robustness reasons.
This feature is available in Postfix 2.8 and later.
A case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that
  the postscreen(8) server will not send in the EHLO response to a remote
  SMTP client. See smtpd_discard_ehlo_keywords for details.
This feature is available in Postfix 2.8 and later.
The action that postscreen(8) takes when a remote SMTP client's combined
  DNSBL score is equal to or greater than a threshold (as defined with the
  postscreen_dnsbl_sites and postscreen_dnsbl_threshold parameters). Specify one
  of the following:
  - ignore (default)
- Ignore the failure of this test. Allow other tests to complete. Repeat
      this test the next time the client connects. This option is useful for
      testing and collecting statistics without blocking mail.
    
 
- enforce
- Allow other tests to complete. Reject attempts to deliver mail with a 550
      SMTP reply, and log the helo/sender/recipient information. Repeat this
      test the next time the client connects.
    
 
- drop
- Drop the connection immediately with a 521 SMTP reply. Repeat this test
      the next time the client connects.
    
 
This feature is available in Postfix 2.8.
Allow a remote SMTP client to skip "before" and "after 220
  greeting" protocol tests, based on its combined DNSBL score as defined
  with the postscreen_dnsbl_sites parameter.
Specify a negative value to enable this feature. When a client
    passes the postscreen_dnsbl_allowlist_threshold without having failed other
    tests, all pending or disabled tests are flagged as completed with a
    time-to-live value equal to postscreen_dnsbl_ttl. When a test was already
    completed, its time-to-live value is updated if it was less than
    postscreen_dnsbl_ttl.
This feature is available in Postfix 3.6 and later.
Available as postscreen_dnsbl_whitelist_threshold in Postfix 2.11
    - 3.5.
The maximum amount of time that postscreen(8) will use the result from a
  successful DNS-based reputation test before a client IP address is required to
  pass that test again. If the DNS reply specifies a shorter TTL value, that
  value will be used unless it would be smaller than postscreen_dnsbl_min_ttl.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is h
    (hours).
This feature is available in Postfix 3.1. The default setting is
    backwards-compatible with older Postfix versions.
The minimum amount of time that postscreen(8) will use the result from a
  successful DNS-based reputation test before a client IP address is required to
  pass that test again. If the DNS reply specifies a larger TTL value, that
  value will be used unless it would be larger than postscreen_dnsbl_max_ttl.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 3.1.
A mapping from an actual DNSBL domain name which includes a secret password, to
  the DNSBL domain name that postscreen will reply with when it rejects mail.
  When no mapping is found, the actual DNSBL domain will be used.
For maximal stability it is best to use a file that is read into
    memory such as pcre:, regexp: or texthash: (texthash: is similar to hash:,
    except a) there is no need to run postmap(1) before the file can be
    used, and b) texthash: does not detect changes after the file is read).
Example:
/etc/postfix/main.cf:
    postscreen_dnsbl_reply_map = texthash:/etc/postfix/dnsbl_reply
/etc/postfix/dnsbl_reply:
   secret.zen.spamhaus.org      zen.spamhaus.org
This feature is available in Postfix 2.8.
Optional list of patterns with DNS allow/denylist domains, filters and weight
  factors. When the list is non-empty, the dnsblog(8) daemon will query
  these domains with the reversed IP addresses of remote SMTP clients, and
  postscreen(8) will update an SMTP client's DNSBL score with each
  non-error reply as described below.
Caution: when postscreen rejects mail, its SMTP response contains
    the DNSBL domain name. Use the postscreen_dnsbl_reply_map feature to hide
    "password" information in DNSBL domain names.
When a client's score is equal to or greater than the threshold
    specified with postscreen_dnsbl_threshold, postscreen(8) can drop the
    connection with the remote SMTP client.
Specify a list of domain=filter*weight patterns, separated by
    comma or whitespace.
  - When a pattern specifies no "=filter", postscreen(8) will
      use any non-error DNSBL query result. Otherwise, postscreen(8) will
      use only DNSBL query results that match the filter. The filter has the
      form d.d.d.d, where each d is a number, or a pattern inside [] that
      contains one or more ";"-separated numbers or number..number
      ranges.
- When a pattern specifies no "*weight", the weight of the pattern
      is 1. Otherwise, the weight must be an integral number. Specify a negative
      number for allowlisting.
- When a pattern matches one or more DNSBL query results,
      postscreen(8) adds that pattern's weight once to the remote SMTP
      client's DNSBL score.
    
 
Examples:
To use example.com as a high-confidence blocklist, and to block
    mail with example.net and example.org only when both agree:
postscreen_dnsbl_threshold = 2
postscreen_dnsbl_sites = example.com*2, example.net, example.org
To filter only DNSBL replies containing 127.0.0.4:
postscreen_dnsbl_sites = example.com=127.0.0.4
This feature is available in Postfix 2.8.
The inclusive lower bound for blocking a remote SMTP client, based on its
  combined DNSBL score as defined with the postscreen_dnsbl_sites parameter.
This feature is available in Postfix 2.8.
The time limit for DNSBL or DNSWL lookups. This is separate from the timeouts in
  the dnsblog(8) daemon which are defined by system resolver(3)
  routines.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 3.0.
The amount of time that postscreen(8) will use the result from a
  successful DNS-based reputation test before a client IP address is required to
  pass that test again.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is h
    (hours).
This feature is available in Postfix 2.8-3.0. It was replaced by
    postscreen_dnsbl_max_ttl in Postfix 3.1.
Renamed to postscreen_dnsbl_allowlist_threshold in Postfix 3.6.
This feature is available in Postfix 2.11 - 3.5.
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and require
  that clients use TLS encryption. See smtpd_postscreen_enforce_tls for details.
This feature is available in Postfix 2.8 and later. Preferably,
    use postscreen_tls_security_level instead.
List of characters that are permitted in postscreen_reject_footer attribute
  expansions. See smtpd_expansion_filter for further details.
This feature is available in Postfix 2.8 and later.
List of commands that the postscreen(8) server considers in violation of
  the SMTP protocol. See smtpd_forbidden_commands for syntax, and
  postscreen_non_smtp_command_action for possible actions.
This feature is available in Postfix 2.8.
The action that postscreen(8) takes when a remote SMTP client speaks
  before its turn within the time specified with the postscreen_greet_wait
  parameter. Specify one of the following:
  - ignore (default)
- Ignore the failure of this test. Allow other tests to complete. Repeat
      this test the next time the client connects. This option is useful for
      testing and collecting statistics without blocking mail.
    
 
- enforce
- Allow other tests to complete. Reject attempts to deliver mail with a 550
      SMTP reply, and log the helo/sender/recipient information. Repeat this
      test the next time the client connects.
    
 
- drop
- Drop the connection immediately with a 521 SMTP reply. Repeat this test
      the next time the client connects.
    
 
In either case, postscreen(8) will not allowlist the remote
    SMTP client IP address.
This feature is available in Postfix 2.8.
The text in the optional "220-text..." server response
  that postscreen(8) sends ahead of the real Postfix SMTP server's
  "220 text..." response, in an attempt to confuse bad SMTP clients so
  that they speak before their turn (pre-greet). Specify an empty value to
  disable this feature.
This feature is available in Postfix 2.8.
The amount of time that postscreen(8) will use the result from a
  successful PREGREET test. During this time, the client IP address is excluded
  from this test. The default is relatively short, because a good client can
  immediately talk to a real Postfix SMTP server.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is d
    (days).
This feature is available in Postfix 2.8.
The amount of time that postscreen(8) will wait for an SMTP client to
  send a command before its turn, and for DNS blocklist lookup results to arrive
  (default: up to 2 seconds under stress, up to 6 seconds otherwise).
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.8.
Require that a remote SMTP client sends HELO or EHLO before commencing a MAIL
  transaction.
This feature is available in Postfix 2.8.
The action that postscreen(8) takes when a remote SMTP client sends
  non-SMTP commands as specified with the postscreen_forbidden_commands
  parameter. Specify one of the following:
  - ignore
- Ignore the failure of this test. Allow other tests to complete. Do
      not repeat this test before the result from some other test
      expires. This option is useful for testing and collecting statistics
      without blocking mail permanently.
    
 
- enforce
- Allow other tests to complete. Reject attempts to deliver mail with a 550
      SMTP reply, and log the helo/sender/recipient information. Repeat this
      test the next time the client connects.
    
 
- drop
- Drop the connection immediately with a 521 SMTP reply. Repeat this test
      the next time the client connects. This action is the same as with the
      Postfix SMTP server's smtpd_forbidden_commands feature.
    
 
This feature is available in Postfix 2.8.
Enable "non-SMTP command" tests in the postscreen(8) server.
  These tests are expensive: a client must disconnect after it passes the test,
  before it can talk to a real Postfix SMTP server.
This feature is available in Postfix 2.8.
The amount of time that postscreen(8) will use the result from a
  successful "non_smtp_command" SMTP protocol test. During this time,
  the client IP address is excluded from this test. The default is long because
  a client must disconnect after it passes the test, before it can talk to a
  real Postfix SMTP server.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is d
    (days).
This feature is available in Postfix 2.8.
The action that postscreen(8) takes when a remote SMTP client sends
  multiple commands instead of sending one command and waiting for the server to
  respond. Specify one of the following:
  - ignore
- Ignore the failure of this test. Allow other tests to complete. Do
      not repeat this test before the result from some other test
      expires. This option is useful for testing and collecting statistics
      without blocking mail permanently.
    
 
- enforce
- Allow other tests to complete. Reject attempts to deliver mail with a 550
      SMTP reply, and log the helo/sender/recipient information. Repeat this
      test the next time the client connects.
    
 
- drop
- Drop the connection immediately with a 521 SMTP reply. Repeat this test
      the next time the client connects.
    
 
This feature is available in Postfix 2.8.
Enable "pipelining" SMTP protocol tests in the postscreen(8)
  server. These tests are expensive: a good client must disconnect after it
  passes the test, before it can talk to a real Postfix SMTP server.
This feature is available in Postfix 2.8.
The amount of time that postscreen(8) will use the result from a
  successful "pipelining" SMTP protocol test. During this time, the
  client IP address is excluded from this test. The default is long because a
  good client must disconnect after it passes the test, before it can talk to a
  real Postfix SMTP server.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is d
    (days).
This feature is available in Postfix 2.8.
The number of clients that can be waiting for service from a real Postfix SMTP
  server process. When this queue is full, all clients will receive a 421
  response.
This feature is available in Postfix 2.8.
The number of non-allowlisted clients that can be waiting for a decision whether
  they will receive service from a real Postfix SMTP server process. When this
  queue is full, all non-allowlisted clients will receive a 421 response.
This feature is available in Postfix 2.8.
Optional information that is appended after a 4XX or 5XX postscreen(8)
  server response. See smtpd_reject_footer for further details.
This feature is available in Postfix 2.8 and later.
Optional lookup table for information that is appended after a 4XX or 5XX
  postscreen(8) server response. See smtpd_reject_footer_maps for further
  details.
This feature is available in Postfix 3.4 and later.
The SMTP TLS security level for the postscreen(8) server; when a
  non-empty value is specified, this overrides the obsolete parameters
  postscreen_use_tls and postscreen_enforce_tls. See smtpd_tls_security_level
  for details.
This feature is available in Postfix 2.8 and later.
The name of the proxy protocol used by an optional before-postscreen proxy
  agent. When a proxy agent is used, this protocol conveys local and remote
  address and port information. Specify "postscreen_upstream_proxy_protocol
  = haproxy" to enable the haproxy protocol; version 2 is supported with
  Postfix 3.5 and later.
This feature is available in Postfix 2.10 and later.
The time limit for the proxy protocol specified with the
  postscreen_upstream_proxy_protocol parameter.
This feature is available in Postfix 2.10 and later.
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not
  require that clients use TLS encryption.
This feature is available in Postfix 2.8 and later. Preferably,
    use postscreen_tls_security_level instead.
How much time a postscreen(8) process may take to respond to a remote
  SMTP client command or to perform a cache operation before it is terminated by
  a built-in watchdog timer. This is a safety mechanism that prevents
  postscreen(8) from becoming non-responsive due to a bug in Postfix
  itself or in system software. To avoid false alarms and unnecessary cache
  corruption this limit cannot be set under 10s.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.8.
Renamed to postscreen_allowlist_interfaces in Postfix 3.6.
This feature is available in Postfix 2.9 - 3.5.
The message delivery contexts where the Postfix local(8) delivery agent
  prepends a Delivered-To: message header with the address that the mail was
  delivered to. This information is used for mail delivery loop detection.
By default, the Postfix local delivery agent prepends a
    Delivered-To: header when forwarding mail and when delivering to file
    (mailbox) and command. Turning off the Delivered-To: header when forwarding
    mail is not recommended.
Specify zero or more of forward, file, or
    command.
Example:
prepend_delivered_header = forward
The process ID of a Postfix command or daemon process.
The location of Postfix PID files relative to $queue_directory. This is a
  read-only parameter.
The process name of a Postfix command or daemon process.
What address lookup tables copy an address extension from the lookup key to the
  lookup result.
For example, with a virtual(5) mapping of
    "joe@example.com => joe.user@example.net", the
    address "joe+foo@example.com" would rewrite to
    "joe.user+foo@example.net".
Specify zero or more of canonical, virtual,
    alias, forward, include or generic. These cause
    address extension propagation with canonical(5), virtual(5),
    and aliases(5) maps, with local(8) .forward and :include: file
    lookups, and with smtp(8) generic maps, respectively.
Note: enabling this feature for types other than canonical
    and virtual is likely to cause problems when mail is forwarded to
    other sites, especially with mail that is sent to a mailing list exploder
    address.
Examples:
propagate_unmatched_extensions = canonical, virtual, alias,
        forward, include
propagate_unmatched_extensions = canonical, virtual
The remote network interface addresses that this mail system receives mail on by
  way of a proxy or network address translation unit.
This feature is available in Postfix 2.0 and later.
You must specify your "outside" proxy/NAT addresses when
    your system is a backup MX host for other domains, otherwise mail delivery
    loops will happen when the primary MX host is down.
Example:
proxy_interfaces = 1.2.3.4
The lookup tables that the proxymap(8) server is allowed to access for
  the read-only service.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Table references that don't begin with
    proxy: are ignored.
This feature is available in Postfix 2.0 and later.
The lookup tables that the proxymap(8) server is allowed to access for
  the read-write service. Postfix-owned local database files should be stored
  under the Postfix-owned data_directory. Table references that don't begin with
  proxy: are ignored.
This feature is available in Postfix 2.5 and later.
The name of the proxymap read-only table lookup service. This service is
  normally implemented by the proxymap(8) daemon.
This feature is available in Postfix 2.6 and later.
The name of the proxywrite read-write table lookup service. This service is
  normally implemented by the proxymap(8) daemon.
This feature is available in Postfix 2.6 and later.
The minimal delay between warnings that a specific destination is clogging up
  the Postfix active queue. Specify 0 to disable.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
This feature is enabled with the helpful_warnings parameter.
This feature is available in Postfix 2.0 and later.
How much time a Postfix queue manager process may take to handle a request
  before it is terminated by a built-in watchdog timer.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.8 and later.
Obsolete feature: the percentage of delivery resources that a busy mail system
  will use up for delivery of a large mailing list message.
This feature exists only in the oqmgr(8) old queue manager.
    The current queue manager solves the problem in a better way.
The time limit for the queue manager to send or receive information over an
  internal communication channel. The purpose is to break out of deadlock
  situations. If the time limit is exceeded the software either retries or
  aborts the operation.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.8 and later.
The maximal number of messages in the active queue.
The maximal number of recipients held in memory by the Postfix queue manager,
  and the maximal size of the short-term, in-memory "dead" destination
  status cache.
The minimal number of in-memory recipients for any message. This takes priority
  over any other in-memory recipient limits (i.e., the global
  qmgr_message_recipient_limit and the per transport _recipient_limit) if
  necessary. The minimum value allowed for this parameter is 1.
What remote QMQP clients are allowed to connect to the Postfix QMQP server port.
By default, no client is allowed to use the service. This is
    because the QMQP server will relay mail to any destination.
Specify a list of client patterns. A list pattern specifies a host
    name, a domain name, an internet address, or a network/mask pattern, where
    the mask specifies the number of bits in the network part. When a pattern
    specifies a file name, its contents are substituted for the file name; when
    a pattern is a "type:table" table specification, table lookup is
    used instead.
Patterns are separated by whitespace and/or commas. In order to
    reverse the result, precede a pattern with an exclamation point (!). The
    form "!/file/name" is supported only in Postfix version 2.4 and
    later.
Pattern matching of domain names is controlled by the presence or
    absence of "qmqpd_authorized_clients" in the
    parent_domain_matches_subdomains parameter value.
Example:
qmqpd_authorized_clients = !192.168.0.1, 192.168.0.0/24
Enable logging of the remote QMQP client port in addition to the hostname and IP
  address. The logging format is "host[address]:port".
This feature is available in Postfix 2.5 and later.
How long the Postfix QMQP server will pause before sending a negative reply to
  the remote QMQP client. The purpose is to slow down confused or malicious
  clients.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
The time limit for sending or receiving information over the network. If a read
  or write operation blocks for more than $qmqpd_timeout seconds the Postfix
  QMQP server gives up and disconnects.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The location of the Postfix top-level queue directory. This is the root
  directory of Postfix daemon processes that run chrooted.
The maximal number of (name=value) attributes that may be stored in a Postfix
  queue file. The limit is enforced by the cleanup(8) server.
This feature is available in Postfix 2.0 and later.
The minimal amount of free space in bytes in the queue file system that is
  needed to receive mail. This is currently used by the Postfix SMTP server to
  decide if it will accept any mail at all.
By default, the Postfix SMTP server rejects MAIL FROM commands
    when the amount of free space is less than 1.5*$message_size_limit (Postfix
    version 2.1 and later). To specify a higher minimum free space limit,
    specify a queue_minfree value that is at least 1.5*$message_size_limit.
With Postfix versions 2.0 and earlier, a queue_minfree value of
    zero means there is no minimum required amount of free space.
The time between deferred queue scans by the queue manager; prior to Postfix 2.4
  the default value was 1000s.
This parameter should be set less than or equal to
    $minimal_backoff_time. See also $maximal_backoff_time.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The name of the qmgr(8) service. This service manages the Postfix queue
  and schedules delivery requests.
This feature is available in Postfix 2.0 and later.
Optional lookup tables with RBL response templates. The tables are indexed by
  the RBL domain name. By default, Postfix uses the default template as
  specified with the default_rbl_reply configuration parameter. See there for a
  discussion of the syntax of RBL reply templates.
This feature is available in Postfix 2.0 and later.
The location of Postfix README files that describe how to build, configure or
  operate a specific Postfix subsystem or feature.
Enable or disable recipient validation, built-in content filtering, or address
  mapping. Typically, these are specified in master.cf as command-line arguments
  for the smtpd(8), qmqpd(8) or pickup(8) daemons.
Specify zero or more of the following options. The options
    override main.cf settings and are either implemented by smtpd(8),
    qmqpd(8), or pickup(8) themselves, or they are forwarded to
    the cleanup server.
  - no_unknown_recipient_checks
- Do not try to reject unknown recipients (SMTP server only). This is
      typically specified AFTER an external content filter.
    
 
- no_address_mappings
- Disable canonical address mapping, virtual alias map expansion, address
      masquerading, and automatic BCC (blind carbon-copy) recipients. This is
      typically specified BEFORE an external content filter.
    
 
- no_header_body_checks
- Disable header/body_checks. This is typically specified AFTER an external
      content filter.
    
 
- no_milters
- Disable Milter (mail filter) applications. This is typically specified
      AFTER an external content filter.
    
 
Note: when the "BEFORE content filter"
    receive_override_options setting is specified in the main.cf file, specify
    the "AFTER content filter" receive_override_options setting in
    master.cf (and vice versa).
Examples:
receive_override_options =
    no_unknown_recipient_checks, no_header_body_checks
receive_override_options = no_address_mappings
This feature is available in Postfix 2.1 and later.
Optional BCC (blind carbon-copy) address lookup tables, indexed by envelope
  recipient address. The BCC address (multiple results are not supported) is
  added when mail enters from outside of Postfix.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
The table search order is as follows:
  - Look up the "user+extension@domain.tld" address including the
      optional address extension.
- Look up the "user@domain.tld" address without the optional
      address extension.
- Look up the "user+extension" address local part when the
      recipient domain equals $myorigin, $mydestination, $inet_interfaces or
      $proxy_interfaces.
- Look up the "user" address local part when the recipient domain
      equals $myorigin, $mydestination, $inet_interfaces or
    $proxy_interfaces.
- Look up the "@domain.tld" part.
    
 
Note: with Postfix 2.3 and later the BCC address is added as if it
    was specified with NOTIFY=NONE. The sender will not be notified when the BCC
    address is undeliverable, as long as all down-stream software implements RFC
    3461.
Note: with Postfix 2.2 and earlier the sender will unconditionally
    be notified when the BCC address is undeliverable.
Note: automatic BCC recipients are produced only for new mail. To
    avoid mailer loops, automatic BCC recipients are not generated after Postfix
    forwards mail internally, or after Postfix generates mail itself.
Example:
recipient_bcc_maps = hash:/etc/postfix/recipient_bcc
After a change, run "postmap
    /etc/postfix/recipient_bcc".
This feature is available in Postfix 2.1 and later.
What addresses are subject to recipient_canonical_maps address mapping. By
  default, recipient_canonical_maps address mapping is applied to envelope
  recipient addresses, and to header recipient addresses.
Specify one or more of: envelope_recipient, header_recipient
This feature is available in Postfix 2.2 and later.
Optional address mapping lookup tables for envelope and header recipient
  addresses. The table format and lookups are documented in canonical(5).
Note: $recipient_canonical_maps is processed before
    $canonical_maps.
Example:
recipient_canonical_maps = hash:/etc/postfix/recipient_canonical
The set of characters that can separate an email address localpart, user name,
  or a .forward file name from its extension. For example, with
  "recipient_delimiter = +", the software tries user+foo@example.com
  before trying user@example.com, user+foo before trying user, and .forward+foo
  before trying .forward.
More formally, an email address localpart or user name is
    separated from its extension by the first character that matches the
    recipient_delimiter set. The delimiter character and extension may then be
    used to generate an extended .forward file name. This implementation
    recognizes one delimiter character and one extension per email address
    localpart or email address. With Postfix 2.10 and earlier, the
    recipient_delimiter specifies a single character.
See canonical(5), local(8), relocated(5) and
    virtual(5) for the effects of recipient_delimiter on lookups in
    aliases, canonical, virtual, and relocated maps, and see the
    propagate_unmatched_extensions parameter for propagating an extension from
    one email address to another.
When used in command_execution_directory, forward_path, or
    luser_relay, ${recipient_delimiter} is replaced with the actual recipient
    delimiter that was found in the recipient email address (Postfix 2.11 and
    later), or it is replaced with the main.cf recipient_delimiter parameter
    value (Postfix 2.10 and earlier).
The recipient_delimiter is not applied to the mailer-daemon
    address, the postmaster address, or the double-bounce address. With the
    default "owner_request_special = yes" setting, the
    recipient_delimiter is also not applied to addresses with the special
    "owner-" prefix or the special "-request" suffix.
Examples:
# Handle Postfix-style extensions.
recipient_delimiter = +
# Handle both Postfix and qmail extensions (Postfix 2.11 and later).
recipient_delimiter = +-
# Use .forward for mail without address extension, and for mail with
# an unrecognized address extension.
forward_path = $home/.forward${recipient_delimiter}${extension},
    $home/.forward
The numerical Postfix SMTP server response code when a remote SMTP client
  request is rejected by the "reject" restriction.
Do not change this unless you have a complete understanding of RFC
    5321.
The Postfix SMTP server's action when a reject-type restriction fails due to a
  temporary error condition. Specify "defer" to defer the remote SMTP
  client request immediately. With the default "defer_if_permit"
  action, the Postfix SMTP server continues to look for opportunities to reject
  mail, and defers the client request only if it would otherwise be accepted.
For finer control, see: unverified_recipient_tempfail_action,
    unverified_sender_tempfail_action, unknown_address_tempfail_action, and
    unknown_helo_hostname_tempfail_action.
This feature is available in Postfix 2.6 and later.
List of tables with remote SMTP client-certificate fingerprints or public key
  fingerprints (Postfix 2.9 and later) for which the Postfix SMTP server will
  allow access with the permit_tls_clientcerts feature. The fingerprint digest
  algorithm is configurable via the smtpd_tls_fingerprint_digest parameter
  (hard-coded as md5 prior to Postfix version 2.5).
The default algorithm is sha256 with Postfix >= 3.6 and
    the compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
    the default algorithm is md5. The best-practice algorithm is now
    sha256. Recent advances in hash function cryptanalysis have led to
    md5 and sha1 being deprecated in favor of sha256. However, as long as there
    are no known "second pre-image" attacks against the older
    algorithms, their use in this context, though not recommended, is still
    likely safe.
Postfix lookup tables are in the form of (key, value) pairs. Since
    we only need the key, the value can be chosen freely, e.g. the name of the
    user or host: D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80
  lutzpc.at.home
Example:
relay_clientcerts = hash:/etc/postfix/relay_clientcerts
For more fine-grained control, use check_ccert_access to select an
    appropriate access(5) policy for each client. See
    RESTRICTION_CLASS_README.
This feature is available with Postfix version 2.2.
The maximal number of parallel deliveries to the same destination via the relay
  message delivery transport. This limit is enforced by the queue manager. The
  message delivery transport name is the first field in the entry in the
  master.cf file.
This feature is available in Postfix 2.0 and later.
The maximal number of recipients per message for the relay message delivery
  transport. This limit is enforced by the queue manager. The message delivery
  transport name is the first field in the entry in the master.cf file.
Setting this parameter to a value of 1 changes the meaning of
    relay_destination_concurrency_limit from concurrency per domain into
    concurrency per recipient.
This feature is available in Postfix 2.0 and later.
What destination domains (and subdomains thereof) this system will relay mail
  to. For details about how the relay_domains value is used, see the description
  of the permit_auth_destination and reject_unauth_destination SMTP recipient
  restrictions.
Domains that match $relay_domains are delivered with the
    $relay_transport mail delivery transport. The SMTP server validates
    recipient addresses with $relay_recipient_maps and rejects non-existent
    recipients. See also the relay domains address class in the
    ADDRESS_CLASS_README file.
Note: Postfix will not automatically forward mail for domains that
    list this system as their primary or backup MX host. See the
    permit_mx_backup restriction in the postconf(5) manual page.
Specify a list of host or domain names, "/file/name"
    patterns or "type:table" lookup tables, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a (parent) domain
    appears as lookup key. Specify "!pattern" to exclude a domain from
    the list. The form "!/file/name" is supported only in Postfix
    version 2.4 and later.
Pattern matching of domain names is controlled by the presence or
    absence of "relay_domains" in the parent_domain_matches_subdomains
    parameter value.
The numerical Postfix SMTP server response code when a client request is
  rejected by the reject_unauth_destination recipient restriction.
Do not change this unless you have a complete understanding of RFC
    5321.
Optional lookup tables with all valid addresses in the domains that match
  $relay_domains. Specify @domain as a wild-card for domains that have no valid
  recipient list, and become a source of backscatter mail: Postfix accepts spam
  for non-existent recipients and then floods innocent people with undeliverable
  mail. Technically, tables listed with $relay_recipient_maps are used as lists:
  Postfix needs to know only if a lookup string is found or not, but it does not
  use the result from the table lookup.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
If this parameter is non-empty, then the Postfix SMTP server will
    reject mail to unknown relay users. This feature is off by default.
See also the relay domains address class in the
    ADDRESS_CLASS_README file.
Example:
relay_recipient_maps = hash:/etc/postfix/relay_recipients
This feature is available in Postfix 2.0 and later.
The default mail delivery transport and next-hop destination for remote delivery
  to domains listed with $relay_domains. In order of decreasing precedence, the
  nexthop destination is taken from $relay_transport,
  $sender_dependent_relayhost_maps, $relayhost, or from the recipient domain.
  This information can be overruled with the transport(5) table.
Specify a string of the form transport:nexthop, where
    transport is the name of a mail delivery transport defined in
    master.cf. The :nexthop destination is optional; its syntax is
    documented in the manual page of the corresponding delivery agent.
See also the relay domains address class in the
    ADDRESS_CLASS_README file.
This feature is available in Postfix 2.0 and later.
The next-hop destination(s) for non-local mail; overrides non-local domains in
  recipient addresses. This information is overruled with relay_transport,
  sender_dependent_default_transport_maps, default_transport,
  sender_dependent_relayhost_maps and with the transport(5) table.
On an intranet, specify the organizational domain name. If your
    internal DNS uses no MX records, specify the name of the intranet gateway
    host instead.
In the case of SMTP or LMTP delivery, specify one or more
    destinations in the form of a domain name, hostname, hostname:port,
    [hostname]:port, [hostaddress] or [hostaddress]:port, separated by comma or
    whitespace. The form [hostname] turns off MX lookups. Multiple destinations
    are supported in Postfix 3.5 and later.
If you're connected via UUCP, see the UUCP_README file for useful
    information.
Examples:
relayhost = $mydomain
relayhost = [gateway.example.com]
relayhost = mail1.example:587, mail2.example:587
relayhost = [an.ip.add.ress]
Optional lookup tables with new contact information for users or domains that no
  longer exist. The table format and lookups are documented in
  relocated(5).
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
If you use this feature, run "postmap
    /etc/postfix/relocated" to build the necessary DBM or DB file after
    change, then "postfix reload" to make the changes
    visible.
Examples:
relocated_maps = dbm:/etc/postfix/relocated
relocated_maps = hash:/etc/postfix/relocated
Rewrite or add message headers in mail from remote clients if the
  remote_header_rewrite_domain parameter value is non-empty, updating incomplete
  addresses with the domain specified in the remote_header_rewrite_domain
  parameter, and adding missing headers.
The local_header_rewrite_clients parameter controls what clients
    Postfix considers local.
Examples:
The safe setting: append "domain.invalid" to incomplete
    header addresses from remote SMTP clients, so that those addresses cannot be
    confused with local addresses.
  
remote_header_rewrite_domain = domain.invalid
The default, purist, setting: don't rewrite headers from remote
    clients at all.
  
remote_header_rewrite_domain =
Require that a local(8) recipient's home directory exists before mail
  delivery is attempted. By default this test is disabled. It can be useful for
  environments that import home directories to the mail server (IMPORTING HOME
  DIRECTORIES IS NOT RECOMMENDED).
Reset the local(8) delivery agent's idea of the owner-alias attribute,
  when delivering mail to a child alias that does not have its own owner alias.
This feature is available in Postfix 2.8 and later. With older
    Postfix releases, the behavior is as if this parameter is set to
    "yes".
As documented in aliases(5), when an alias name has
    a companion alias named owner-name, this will replace the envelope
    sender address, so that delivery errors will be reported to the owner alias
    instead of the sender. This configuration is recommended for mailing
  lists.
A less known property of the owner alias is that it also forces
    the local(8) delivery agent to write local and remote addresses from
    alias expansion to a new queue file, instead of attempting to deliver mail
    to local addresses as soon as they come out of alias expansion.
Writing local addresses from alias expansion to a new queue file
    allows for robust handling of temporary delivery errors: errors with one
    local member have no effect on deliveries to other members of the list. On
    the other hand, delivery to local addresses as soon as they come out of
    alias expansion is fragile: a temporary error with one local address from
    alias expansion will cause the entire alias to be expanded repeatedly until
    the error goes away, or until the message expires in the queue. In that
    case, a problem with one list member results in multiple message deliveries
    to other list members.
The default behavior of Postfix 2.8 and later is to keep the
    owner-alias attribute of the parent alias, when delivering mail to a child
    alias that does not have its own owner alias. Then, local addresses from
    that child alias will be written to a new queue file, and a temporary error
    with one local address will not affect delivery to other mailing list
    members.
Unfortunately, older Postfix releases reset the owner-alias
    attribute when delivering mail to a child alias that does not have its own
    owner alias. To be precise, this resets only the decision to create a new
    queue file, not the decision to override the envelope sender address. The
    local(8) delivery agent then attempts to deliver local addresses as
    soon as they come out of child alias expansion. If delivery to any address
    from child alias expansion fails with a temporary error condition, the
    entire mailing list may be expanded repeatedly until the mail expires in the
    queue, resulting in multiple deliveries of the same message to mailing list
    members.
Resolve a recipient address safely instead of correctly, by looking inside
  quotes.
By default, the Postfix address resolver does not quote the
    address localpart as per RFC 822, so that additional @ or % or ! operators
    remain visible. This behavior is safe but it is also technically
  incorrect.
If you specify "resolve_dequoted_address = no", then the
    Postfix resolver will not know about additional @ etc. operators in the
    address localpart. This opens opportunities for obscure mail relay attacks
    with user@domain@domain addresses when Postfix provides backup MX service
    for Sendmail systems.
Resolve an address that ends in the "@" null domain as if the local
  hostname were specified, instead of rejecting the address as invalid.
This feature is available in Postfix 2.1 and later. Earlier
    versions always resolve the null domain as the local hostname.
The Postfix SMTP server uses this feature to reject mail from or
    to addresses that end in the "@" null domain, and from addresses
    that rewrite into a form that ends in the "@" null domain.
Resolve "user@ipaddress" as "user@[ipaddress]", instead of
  rejecting the address as invalid.
This feature is available in Postfix 2.3 and later.
Avoid logging that implies white is better than black. Instead use 'allowlist',
  'denylist', and variations of those words.
This feature is available in Postfix 3.6 and later.
The name of the address rewriting service. This service rewrites addresses to
  standard form and resolves them to a (delivery method, next-hop host,
  recipient) triple.
This feature is available in Postfix 2.0 and later.
The name of the directory with example Postfix configuration files. Starting
  with Postfix 2.1, these files have been replaced with the postconf(5)
  manual page.
When authenticating to a remote SMTP or LMTP server with the default setting
  "no", send no SASL authoriZation ID (authzid); send only the SASL
  authentiCation ID (authcid) plus the authcid's password.
The non-default setting "yes" enables the behavior of
    older Postfix versions. These always send a SASL authzid that is equal to
    the SASL authcid, but this causes interoperability problems with some SMTP
    servers.
This feature is available in Postfix 2.4.4 and later.
This parameter should not be used. It was replaced by
  sender_dependent_relayhost_maps in Postfix version 2.3.
Optional BCC (blind carbon-copy) address lookup tables, indexed by envelope
  sender address. The BCC address (multiple results are not supported) is added
  when mail enters from outside of Postfix.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
The table search order is as follows:
  - Look up the "user+extension@domain.tld" address including the
      optional address extension.
- Look up the "user@domain.tld" address without the optional
      address extension.
- Look up the "user+extension" address local part when the sender
      domain equals $myorigin, $mydestination, $inet_interfaces or
      $proxy_interfaces.
- Look up the "user" address local part when the sender domain
      equals $myorigin, $mydestination, $inet_interfaces or
    $proxy_interfaces.
- Look up the "@domain.tld" part.
    
 
Note: with Postfix 2.3 and later the BCC address is added as if it
    was specified with NOTIFY=NONE. The sender will not be notified when the BCC
    address is undeliverable, as long as all down-stream software implements RFC
    3461.
Note: with Postfix 2.2 and earlier the sender will be notified
    when the BCC address is undeliverable.
Note: automatic BCC recipients are produced only for new mail. To
    avoid mailer loops, automatic BCC recipients are not generated after Postfix
    forwards mail internally, or after Postfix generates mail itself.
Example:
sender_bcc_maps = hash:/etc/postfix/sender_bcc
After a change, run "postmap
    /etc/postfix/sender_bcc".
This feature is available in Postfix 2.1 and later.
What addresses are subject to sender_canonical_maps address mapping. By default,
  sender_canonical_maps address mapping is applied to envelope sender addresses,
  and to header sender addresses.
Specify one or more of: envelope_sender, header_sender
This feature is available in Postfix 2.2 and later.
Optional address mapping lookup tables for envelope and header sender addresses.
  The table format and lookups are documented in canonical(5).
Example: you want to rewrite the SENDER address
    "user@ugly.example" to "user@pretty.example", while
    still being able to send mail to the RECIPIENT address
    "user@ugly.example".
Note: $sender_canonical_maps is processed before
  $canonical_maps.
Example:
sender_canonical_maps = hash:/etc/postfix/sender_canonical
A sender-dependent override for the global default_transport parameter setting.
  The tables are searched by the envelope sender address and @domain. A lookup
  result of DUNNO terminates the search without overriding the global
  default_transport parameter setting. This information is overruled with the
  transport(5) table.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
Note: this overrides default_transport, not transport_maps, and
    therefore the expected syntax is that of default_transport, not the syntax
    of transport_maps. Specifically, this does not support the transport_maps
    syntax for null transport, null nexthop, or null email addresses.
For safety reasons, this feature does not allow $number
    substitutions in regular expression maps.
This feature is available in Postfix 2.7 and later.
A sender-dependent override for the global relayhost parameter setting. The
  tables are searched by the envelope sender address and @domain. A lookup
  result of DUNNO terminates the search without overriding the global relayhost
  parameter setting (Postfix 2.6 and later). This information is overruled with
  relay_transport, sender_dependent_default_transport_maps, default_transport
  and with the transport(5) table.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
For safety reasons, this feature does not allow $number
    substitutions in regular expression maps.
This feature is available in Postfix 2.3 and later.
Controls how the Postfix sendmail command converts email message line endings
  from <CR><LF> into UNIX format (<LF>).
  - always
- Always convert message lines ending in <CR><LF>. This setting
      is the default with Postfix 2.9 and later.
    
 
- strict
- Convert message lines ending in <CR><LF> only if the first
      input line ends in <CR><LF>. This setting is
      backwards-compatible with Postfix 2.8 and earlier.
    
 
- never
- Never convert message lines ending in <CR><LF>. This setting
      exists for completeness only.
    
 
This feature is available in Postfix 2.9 and later.
A Sendmail compatibility feature that specifies the location of the Postfix
  sendmail(1) command. This command can be used to submit mail into the
  Postfix queue.
The master.cf service name of a Postfix daemon process. This can be used to
  distinguish the logging from different services that use the same program
  name.
Example master.cf entries:
# Distinguish inbound MTA logging from submission and smtps logging.
smtp      inet  n       -       n       -       -       smtpd
submission inet n       -       n       -       -       smtpd
    -o syslog_name=postfix/$service_name
smtps     inet  n       -       n       -       -       smtpd
    -o syslog_name=postfix/$service_name
# Distinguish outbound MTA logging from inbound relay logging.
smtp      unix  -       -       n       -       -       smtp
relay     unix  -       -       n       -       -       smtp
    -o syslog_name=postfix/$service_name
How long the Postfix master(8) waits before forking a server that appears
  to be malfunctioning.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The group ownership of set-gid Postfix commands and of group-writable Postfix
  directories. When this parameter value is changed you need to re-run
  "postfix set-permissions" (with Postfix version 2.0 and
  earlier: "/etc/postfix/post-install set-permissions".
The location of Postfix dynamically-linked libraries (libpostfix-*.so), and the
  default location of Postfix database plugins (postfix-*.so) that have a
  relative pathname in the dynamicmaps.cf file. The shlib_directory parameter
  defaults to "no" when Postfix dynamically-linked libraries and
  database plugins are disabled at compile time, otherwise it typically defaults
  to /usr/lib/postfix or /usr/local/lib/postfix.
Notes:
  - The directory specified with shlib_directory should contain only
      Postfix-related files. Postfix dynamically-linked libraries and database
      plugins should not be installed in a "public" system directory
      such as /usr/lib or /usr/local/lib. Linking Postfix dynamically-linked
      library files or database plugins into non-Postfix programs is not
      supported. Postfix dynamically-linked libraries and database plugins
      implement a Postfix-internal API that changes without maintaining
      compatibility.
- You can change the shlib_directory value after Postfix is built. However,
      you may have to run ldconfig or equivalent to prevent Postfix programs
      from failing because the libpostfix-*.so files are not found. No ldconfig
      command is needed if you keep the libpostfix-*.so files in the compiled-in
      default $shlib_directory location.
    
 
This feature is available in Postfix 3.0 and later.
Display the name of the recipient table in the "User unknown"
  responses. The extra detail makes troubleshooting easier but also reveals
  information that is nobody else's business.
This feature is available in Postfix 2.0 and later.
The name of the showq(8) service. This service produces mail queue status
  reports.
This feature is available in Postfix 2.0 and later.
The address type ("ipv6", "ipv4" or "any") that
  the Postfix SMTP client will try first, when a destination has IPv6 and IPv4
  addresses with equal MX preference. This feature has no effect unless the
  inet_protocols setting enables both IPv4 and IPv6.
Postfix SMTP client address preference has evolved. With Postfix
    2.8 the default is "ipv6"; earlier implementations are hard-coded
    to prefer IPv6 over IPv4.
Notes for mail delivery between sites that have both IPv4 and IPv6
    connectivity:
  - The setting "smtp_address_preference = ipv6" is unsafe. It can
      fail to deliver mail when there is an outage that affects IPv6, while the
      destination is still reachable over IPv4.
- The setting "smtp_address_preference = any" is safe. With this,
      mail will eventually be delivered even if there is an outage that affects
      IPv6 or IPv4, as long as it does not affect both.
    
 
This feature is available in Postfix 2.8 and later.
In the context of email address verification, the SMTP protocol stage that
  determines whether an email address is deliverable. Specify one of
  "rcpt" or "data". The latter is needed with remote SMTP
  servers that reject recipients after the DATA command. Use transport_maps to
  apply this feature selectively:
  
/etc/postfix/main.cf:
    transport_maps = hash:/etc/postfix/transport
  
/etc/postfix/transport:
    smtp-domain-that-verifies-after-data    smtp-data-target:
    lmtp-domain-that-verifies-after-data    lmtp-data-target:
  
/etc/postfix/master.cf:
    smtp-data-target    unix    -    -    n    -    -    smtp
        -o smtp_address_verify_target=data
    lmtp-data-target    unix    -    -    n    -    -    lmtp
        -o lmtp_address_verify_target=data
Unselective use of the "data" target does no harm, but
    will result in unnecessary "lost connection after DATA" events at
    remote SMTP/LMTP servers.
This feature is available in Postfix 3.0 and later.
Always send EHLO at the start of an SMTP session.
With "smtp_always_send_ehlo = no", the Postfix SMTP
    client sends EHLO only when the word "ESMTP" appears in the server
    greeting banner (example: 220 spike.porcupine.org ESMTP Postfix).
When a remote destination resolves to a combination of IPv4 and IPv6 addresses,
  ensure that the Postfix SMTP client can try both address types before it runs
  into the smtp_mx_address_limit.
This avoids an interoperability problem when a destination
    resolves to primarily IPv6 addresses, the smtp_address_limit feature
    eliminates most or all IPv4 addresses, and the destination is not reachable
    over IPv6.
This feature is available in Postfix 3.3 and later.
An optional numerical network address that the Postfix SMTP client should bind
  to when making an IPv4 connection.
This can be specified in the main.cf file for all SMTP clients, or
    it can be specified in the master.cf file for a specific client, for
    example:
  
/etc/postfix/master.cf:
    smtp ... smtp -o smtp_bind_address=11.22.33.44
See smtp_bind_address_enforce for how Postfix should handle errors
    (Postfix 3.7 and later).
Note 1: when inet_interfaces specifies no more than one IPv4
    address, and that address is a non-loopback address, it is automatically
    used as the smtp_bind_address. This supports virtual IP hosting, but can be
    a problem on multi-homed firewalls. See the inet_interfaces documentation
    for more detail.
Note 2: address information may be enclosed inside [], but this
    form is not required here.
An optional numerical network address that the Postfix SMTP client should bind
  to when making an IPv6 connection.
This feature is available in Postfix 2.2 and later.
This can be specified in the main.cf file for all SMTP clients, or
    it can be specified in the master.cf file for a specific client, for
    example:
  
/etc/postfix/master.cf:
    smtp ... smtp -o smtp_bind_address6=1:2:3:4:5:6:7:8
See smtp_bind_address_enforce for how Postfix should handle errors
    (Postfix 3.7 and later).
Note 1: when inet_interfaces specifies no more than one IPv6
    address, and that address is a non-loopback address, it is automatically
    used as the smtp_bind_address6. This supports virtual IP hosting, but can be
    a problem on multi-homed firewalls. See the inet_interfaces documentation
    for more detail.
Note 2: address information may be enclosed inside [], but this
    form is not recommended here.
Defer delivery when the Postfix SMTP client cannot apply the smtp_bind_address
  or smtp_bind_address6 setting. By default, the Postfix SMTP client will
  continue delivery after logging a warning.
This feature is available in Postfix 3.7 and later.
Restricted body_checks(5) tables for the Postfix SMTP client. These
  tables are searched while mail is being delivered. Actions that change the
  delivery time or destination are not available.
This feature is available in Postfix 2.5 and later.
When the remote SMTP servername is a DNS CNAME, replace the servername with the
  result from CNAME expansion for the purpose of logging, SASL password lookup,
  TLS policy decisions, or TLS certificate verification. The value
  "no" hardens Postfix smtp_tls_per_site hostname-based policies
  against false hostname information in DNS CNAME records, and makes SASL
  password file lookups more predictable. This is the default setting as of
  Postfix 2.3.
When DNS CNAME records are validated with secure DNS lookups
    (smtp_dns_support_level = dnssec), they are always allowed to override the
    above servername (Postfix 2.11 and later).
This feature is available in Postfix 2.2.9 and later.
The Postfix SMTP client time limit for completing a TCP connection, or zero (use
  the operating system built-in time limit).
When no connection can be made within the deadline, the Postfix
    SMTP client tries the next address on the mail exchanger list. Specify 0 to
    disable the time limit (i.e. use whatever timeout is implemented by the
    operating system).
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
Permanently enable SMTP connection caching for the specified destinations. With
  SMTP connection caching, a connection is not closed immediately after
  completion of a mail transaction. Instead, the connection is kept open for up
  to $smtp_connection_cache_time_limit seconds. This allows connections to be
  reused for other deliveries, and can improve mail delivery performance.
Specify a comma or white space separated list of destinations or
    pseudo-destinations:
  - if mail is sent without a relay host: a domain name (the right-hand side
      of an email address, without the [] around a numeric IP address),
- if mail is sent via a relay host: a relay host name (without [] or
      non-default TCP port), as specified in main.cf or in the transport
    map,
- if mail is sent via a UNIX-domain socket: a pathname (without the unix:
      prefix),
- a /file/name with domain names and/or relay host names as defined
    above,
- a "type:table" with domain names and/or relay host names on the
      left-hand side. The right-hand side result from "type:table"
      lookups is ignored.
    
 
This feature is available in Postfix 2.2 and later.
Temporarily enable SMTP connection caching while a destination has a high volume
  of mail in the active queue. With SMTP connection caching, a connection is not
  closed immediately after completion of a mail transaction. Instead, the
  connection is kept open for up to $smtp_connection_cache_time_limit seconds.
  This allows connections to be reused for other deliveries, and can improve
  mail delivery performance.
This feature is available in Postfix 2.2 and later.
When SMTP connection caching is enabled, the amount of time that an unused SMTP
  client socket is kept open before it is closed. Do not specify larger values
  without permission from the remote sites.
This feature is available in Postfix 2.2 and later.
When SMTP connection caching is enabled, the number of times that an SMTP
  session may be reused before it is closed, or zero (no limit). With a reuse
  count limit of N, a connection is used up to N+1 times.
NOTE: This feature is unsafe. When a high-volume destination has
    multiple inbound MTAs, then the slowest inbound MTA will attract the most
    connections to that destination. This limitation does not exist with the
    smtp_connection_reuse_time_limit feature.
This feature is available in Postfix 2.11.
The amount of time during which Postfix will use an SMTP connection repeatedly.
  The timer starts when the connection is initiated (i.e. it includes the
  connect, greeting and helo latency, in addition to the latencies of subsequent
  mail delivery transactions).
This feature addresses a performance stability problem with remote
    SMTP servers. This problem is not specific to Postfix: it can happen when
    any MTA sends large amounts of SMTP email to a site that has multiple MX
    hosts.
The problem starts when one of a set of MX hosts becomes slower
    than the rest. Even though SMTP clients connect to fast and slow MX hosts
    with equal probability, the slow MX host ends up with more simultaneous
    inbound connections than the faster MX hosts, because the slow MX host needs
    more time to serve each client request.
The slow MX host becomes a connection attractor. If one MX host
    becomes N times slower than the rest, it dominates mail delivery latency
    unless there are more than N fast MX hosts to counter the effect. And if the
    number of MX hosts is smaller than N, the mail delivery latency becomes
    effectively that of the slowest MX host divided by the total number of MX
    hosts.
The solution uses connection caching in a way that differs from
    Postfix version 2.2. By limiting the amount of time during which a
    connection can be used repeatedly (instead of limiting the number of
    deliveries over that connection), Postfix not only restores fairness in the
    distribution of simultaneous connections across a set of MX hosts, it also
    favors deliveries over connections that perform well, which is exactly what
    we want.
The default reuse time limit, 300s, is comparable to the various
    smtp transaction timeouts which are fair estimates of maximum excess latency
    for a slow delivery. Note that hosts may accept thousands of messages over a
    single connection within the default connection reuse time limit. This
    number is much larger than the default Postfix version 2.2 limit of 10
    messages per cached connection. It may prove necessary to lower the limit to
    avoid interoperability issues with MTAs that exhibit bugs when many messages
    are delivered via a single connection. A lower reuse time limit risks losing
    the benefit of connection reuse when the average connection and mail
    delivery latency exceeds the reuse time limit.
This feature is available in Postfix 2.3 and later.
The Postfix SMTP client time limit for sending the SMTP ".", and for
  receiving the remote SMTP server response.
When no response is received within the deadline, a warning is
    logged that the mail may be delivered multiple times.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
The Postfix SMTP client time limit for sending the SMTP DATA command, and for
  receiving the remote SMTP server response.
Time units: s (seconds), m (minutes), h (hours), d (days), w
    (weeks). The default time unit is s (seconds).
The Postfix SMTP client time limit for sending the SMTP message content. When
  the connection makes no progress for more than $smtp_data_xfer_timeout seconds
  the Postfix SMTP client terminates the transfer.
Time units: s (seconds), m (minutes), h (hours), d (days), w
    (weeks). The default time unit is s (seconds).
Defer mail delivery when no MX record resolves to an IP address.
The default (no) is to return the mail as undeliverable. With
    older Postfix versions the default was to keep trying to deliver the mail
    until someone fixed the MX record or until the mail was too old.
Note: the Postfix SMTP client always ignores MX records with equal
    or worse preference than the local MTA itself.
This feature is available in Postfix 2.1 and later.
Optional filter for the smtp(8) delivery agent to change the delivery
  status code or explanatory text of successful or unsuccessful deliveries. See
  default_delivery_status_filter for details.
NOTE: This feature modifies Postfix SMTP client error or non-error
    messages that may or may not be derived from remote SMTP server responses.
    In contrast, the smtp_reply_filter feature modifies remote SMTP server
    responses only.
The maximal number of parallel deliveries to the same destination via the smtp
  message delivery transport. This limit is enforced by the queue manager. The
  message delivery transport name is the first field in the entry in the
  master.cf file.
The maximal number of recipients per message for the smtp message delivery
  transport. This limit is enforced by the queue manager. The message delivery
  transport name is the first field in the entry in the master.cf file.
Setting this parameter to a value of 1 changes the meaning of
    smtp_destination_concurrency_limit from concurrency per domain into
    concurrency per recipient.
Lookup tables, indexed by the remote SMTP server address, with case insensitive
  lists of EHLO keywords (pipelining, starttls, auth, etc.) that the Postfix
  SMTP client will ignore in the EHLO response from a remote SMTP server. See
  smtp_discard_ehlo_keywords for details. The table is not indexed by hostname
  for consistency with smtpd_discard_ehlo_keyword_address_maps.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
This feature is available in Postfix 2.2 and later.
A case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that
  the Postfix SMTP client will ignore in the EHLO response from a remote SMTP
  server.
This feature is available in Postfix 2.2 and later.
Notes:
  - Specify the silent-discard pseudo keyword to prevent this action
      from being logged.
- Use the smtp_discard_ehlo_keyword_address_maps feature to discard EHLO
      keywords selectively.
    
 
Optional filter for Postfix SMTP client DNS lookup results. Specify zero or more
  lookup tables. The lookup tables are searched in the given order for a match
  with the DNS lookup result, converted to the following form:
    name ttl class type preference value
The class field is always "IN", the
    preference field exists only for MX records, the names of hosts,
    domains, etc. end in ".", and those names are in ASCII form
    (xn--mumble form in the case of UTF8 names).
When a match is found, the table lookup result specifies an
    action. By default, the table query and the action name are
    case-insensitive. Currently, only the IGNORE action is
  implemented.
Notes:
  - Postfix DNS reply filters have no effect on implicit DNS lookups through
      nsswitch.conf or equivalent mechanisms.
- The Postfix SMTP/LMTP client uses smtp_dns_reply_filter and
      lmtp_dns_reply_filter only to discover a remote SMTP or LMTP service
      (record types MX, A, AAAA, and TLSA). These lookups are also made to
      implement the features reject_unverified_sender and
      reject_unverified_recipient.
- The Postfix SMTP/LMTP client defers mail delivery when a filter removes
      all lookup results from a successful query.
- Postfix SMTP server uses smtpd_dns_reply_filter only to look up MX, A,
      AAAA, and TXT records to implement the features
      reject_unknown_helo_hostname, reject_unknown_sender_domain,
      reject_unknown_recipient_domain, reject_rbl_*, and reject_rhsbl_*.
- The Postfix SMTP server logs a warning or defers mail delivery when a
      filter removes all lookup results from a successful query.
    
 
Example: ignore Google AAAA records in Postfix SMTP client DNS
    lookups, because Google sometimes hard-rejects mail from IPv6 clients with
    valid PTR etc. records.
/etc/postfix/main.cf:
    smtp_dns_reply_filter = pcre:/etc/postfix/smtp_dns_reply_filter
/etc/postfix/smtp_dns_reply_filter:
    # /domain ttl IN AAAA address/ action, all case-insensitive.
    # Note: the domain name ends in ".".
    /^\S+\.google\.com\.\s+\S+\s+\S+\s+AAAA\s+/ IGNORE
This feature is available in Postfix 3.0 and later.
DNS Resolver options for the Postfix SMTP client. Specify zero or more of the
  following options, separated by comma or whitespace. Option names are
  case-sensitive. Some options refer to domain names that are specified in the
  file /etc/resolv.conf or equivalent.
  - res_defnames
- Append the current domain name to single-component names (those that do
      not contain a "." character). This can produce incorrect
      results, and is the hard-coded behavior prior to Postfix 2.8.
    
 
- res_dnsrch
- Search for host names in the current domain and in parent domains. This
      can produce incorrect results and is therefore not recommended.
    
 
This feature is available in Postfix 2.8 and later.
Level of DNS support in the Postfix SMTP client. With
  "smtp_dns_support_level" left at its empty default value, the legacy
  "disable_dns_lookups" parameter controls whether DNS is enabled in
  the Postfix SMTP client, otherwise the legacy parameter is ignored.
Specify one of the following:
  - disabled
- Disable DNS lookups. No MX lookups are performed and hostname to address
      lookups are unconditionally "native". This setting is not
      appropriate for hosts that deliver mail to the public Internet. Some
      obsolete how-to documents recommend disabling DNS lookups in some
      configurations with content_filters. This is no longer required and
      strongly discouraged.
    
 
- enabled
- Enable DNS lookups. Nexthop destination domains not enclosed in
      "[]" will be subject to MX lookups. If "dns" and
      "native" are included in the "smtp_host_lookup"
      parameter value, DNS will be queried first to resolve MX-host A records,
      followed by "native" lookups if no answer is found in DNS.
    
 
- dnssec
- Enable DNSSEC lookups. The "dnssec" setting differs from the
      "enabled" setting above in the following ways:
  - Any MX lookups will set RES_USE_DNSSEC and RES_USE_EDNS0 to request
      DNSSEC-validated responses. If the MX response is DNSSEC-validated the
      corresponding hostnames are considered validated.
- The address lookups of validated hostnames are also validated, (provided
      of course "smtp_host_lookup" includes "dns", see
      below).
- Temporary failures in DNSSEC-enabled hostname-to-address resolution block
      any "native" lookups. Additional "native" lookups only
      happen when DNSSEC lookups hard-fail (NODATA or NXDOMAIN).
    
 
The Postfix SMTP client considers non-MX "[nexthop]" and
    "[nexthop]:port" destinations equivalent to statically-validated
    MX records of the form "nexthop. IN MX 0 nexthop." Therefore, with
    "dnssec" support turned on, validated hostname-to-address lookups
    apply to the nexthop domain of any "[nexthop]" or
    "[nexthop]:port" destination. This is also true for LMTP
    "inet:host" and "inet:host:port" destinations, as LMTP
    hostnames are never subject to MX lookups.
The "dnssec" setting is recommended only if you plan to
    use the dane or dane-only TLS security level, otherwise enabling DNSSEC
    support in Postfix offers no additional security. Postfix DNSSEC support
    relies on an upstream recursive nameserver that validates DNSSEC signatures.
    Such a DNS server will always filter out forged DNS responses, even when
    Postfix itself is not configured to use DNSSEC.
When using Postfix DANE support the "smtp_host_lookup"
    parameter should include "dns", as DANE is not applicable to hosts
    resolved via "native" lookups.
As mentioned above, Postfix is not a validating stub resolver; it
    relies on the system's configured DNSSEC-validating recursive nameserver to
    perform all DNSSEC validation. Since this nameserver's DNSSEC-validated
    responses will be fully trusted, it is strongly recommended that the MTA
    host have a local DNSSEC-validating recursive caching nameserver listening
    on a loopback address, and be configured to use only this nameserver for all
    lookups. Otherwise, Postfix may remain subject to man-in-the-middle attacks
    that forge responses from the recursive nameserver
DNSSEC support requires a version of Postfix compiled against a
    reasonably-modern DNS resolver(3) library that implements the
    RES_USE_DNSSEC and RES_USE_EDNS0 resolver options.
This feature is available in Postfix 2.11 and later.
Enforcement mode: require that remote SMTP servers use TLS encryption, and never
  send mail in the clear. This also requires that the remote SMTP server
  hostname matches the information in the remote server certificate, and that
  the remote SMTP server certificate was issued by a CA that is trusted by the
  Postfix SMTP client. If the certificate doesn't verify or the hostname doesn't
  match, delivery is deferred and mail stays in the queue.
The server hostname is matched against all names provided as
    dNSNames in the SubjectAlternativeName. If no dNSNames are specified, the
    CommonName is checked. The behavior may be changed with the
    smtp_tls_enforce_peername option.
This option is useful only if you are definitely sure that you
    will only connect to servers that support RFC 2487 _and_ that provide valid
    server certificates. Typical use is for clients that send all their email to
    a dedicated mailhub.
This feature is available in Postfix 2.2 and later. With Postfix
    2.3 and later use smtp_tls_security_level instead.
Optional list of relay destinations that will be used when an SMTP destination
  is not found, or when delivery fails due to a non-permanent error. With
  Postfix 2.2 and earlier this parameter is called fallback_relay.
By default, smtp_fallback_relay is empty, mail is returned to the
    sender when a destination is not found, and delivery is deferred after it
    fails due to a non-permanent error.
With bulk email deliveries, it can be beneficial to run the
    fallback relay MTA on the same host, so that it can reuse the sender IP
    address. This speeds up deliveries that are delayed by IP-based reputation
    systems (greylist, etc.).
The fallback relays must be SMTP destinations. Specify a domain,
    host, host:port, [host]:port, [address] or [address]:port; the form [host]
    turns off MX lookups. If you specify multiple SMTP destinations, Postfix
    will try them in the specified order.
To prevent mailer loops between MX hosts and fall-back hosts,
    Postfix version 2.2 and later will not use the fallback relays for
    destinations that it is MX host for (assuming DNS lookup is turned on).
Optional lookup tables that perform address rewriting in the Postfix SMTP
  client, typically to transform a locally valid address into a globally valid
  address when sending mail across the Internet. This is needed when the local
  machine does not have its own Internet domain name, but uses something like
  localdomain.local instead.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
The table format and lookups are documented in generic(5);
    examples are shown in the ADDRESS_REWRITING_README and
    STANDARD_CONFIGURATION_README documents.
This feature is available in Postfix 2.2 and later.
Restricted header_checks(5) tables for the Postfix SMTP client. These
  tables are searched while mail is being delivered. Actions that change the
  delivery time or destination are not available.
This feature is available in Postfix 2.5 and later.
The hostname to send in the SMTP HELO or EHLO command.
The default value is the machine hostname. Specify a hostname or
    [ip.add.re.ss].
This information can be specified in the main.cf file for all SMTP
    clients, or it can be specified in the master.cf file for a specific client,
    for example:
  
/etc/postfix/master.cf:
    mysmtp ... smtp -o smtp_helo_name=foo.bar.com
This feature is available in Postfix 2.0 and later.
The Postfix SMTP client time limit for sending the HELO or EHLO command, and for
  receiving the initial remote SMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
What mechanisms the Postfix SMTP client uses to look up a host's IP address.
  This parameter is ignored when DNS lookups are disabled (see:
  disable_dns_lookups and smtp_dns_support_level). The "dns" mechanism
  is always tried before "native" if both are listed.
Specify one of the following:
  - dns
- Hosts can be found in the DNS (preferred).
    
 
- native
- Use the native naming service only (nsswitch.conf, or equivalent
      mechanism).
    
 
- dns, native
- Use the native service for hosts not found in the DNS.
    
 
This feature is available in Postfix 2.1 and later.
The maximal length of message header and body lines that Postfix will send via
  SMTP. This limit does not include the <CR><LF> at the end of each
  line. Longer lines are broken by inserting
  "<CR><LF><SPACE>", to minimize the damage to MIME
  formatted mail. Specify zero to disable this limit.
The Postfix limit of 998 characters not including
    <CR><LF> is consistent with the SMTP limit of 1000 characters
    including <CR><LF>. The Postfix limit was 990 with Postfix 2.8
    and earlier.
The Postfix SMTP client time limit for sending the MAIL FROM command, and for
  receiving the remote SMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
Restricted mime_header_checks(5) tables for the Postfix SMTP client.
  These tables are searched while mail is being delivered. Actions that change
  the delivery time or destination are not available.
This feature is available in Postfix 2.5 and later.
The minimum plaintext data transfer rate in bytes/second for DATA requests, when
  deadlines are enabled with smtp_per_request_deadline. After a write operation
  transfers N plaintext message bytes (possibly after TLS encryption), and after
  the DATA request deadline is decremented by the elapsed time of that write
  operation, the DATA request deadline is incremented by N/smtp_min_data_rate
  seconds. However, the deadline will never be incremented beyond the time limit
  specified with smtp_data_xfer_timeout.
This feature is available in Postfix 3.7 and later.
The maximal number of MX (mail exchanger) IP addresses that can result from
  Postfix SMTP client mail exchanger lookups, or zero (no limit). Prior to
  Postfix version 2.3, this limit was disabled by default.
This feature is available in Postfix 2.1 and later.
The maximal number of SMTP sessions per delivery request before the Postfix SMTP
  client gives up or delivers to a fall-back relay host, or zero (no limit).
  This restriction ignores sessions that fail to complete the SMTP initial
  handshake (Postfix version 2.2 and earlier) or that fail to complete the EHLO
  and TLS handshake (Postfix version 2.3 and later).
This feature is available in Postfix 2.1 and later.
Restricted nested_header_checks(5) tables for the Postfix SMTP client.
  These tables are searched while mail is being delivered. Actions that change
  the delivery time or destination are not available.
This feature is available in Postfix 2.5 and later.
Never send EHLO at the start of an SMTP session. See also the
  smtp_always_send_ehlo parameter.
Change the behavior of the smtp_*_timeout time limits, from a time limit per
  read or write system call, to a time limit to send or receive a complete
  record (an SMTP command line, SMTP response line, SMTP message content line,
  or TLS protocol message). This limits the impact from hostile peers that
  trickle data one byte at a time.
Note: when per-record deadlines are enabled, a short timeout may
    cause problems with TLS over very slow network connections. The reasons are
    that a TLS protocol message can be up to 16 kbytes long (with TLSv1), and
    that an entire TLS protocol message must be sent or received within the
    per-record deadline.
This feature is available in Postfix 2.9-3.6. With older Postfix
    releases, the behavior is as if this parameter is set to "no".
    Postfix 3.7 and later use smtp_per_request_deadline.
Change the behavior of the smtp_*_timeout time limits, from a time limit per
  plaintext or TLS read or write call, to a combined time limit for sending a
  complete SMTP request and for receiving a complete SMTP response. The deadline
  limits only the time spent waiting for plaintext or TLS read or write calls,
  not time spent elsewhere. The per-request deadline limits the impact from
  hostile peers that trickle data one byte at a time.
See smtp_min_data_rate for how the per-request deadline is managed
    during the DATA phase.
Note: when per-request deadlines are enabled, a short time limit
    may cause problems with TLS over very slow network connections. The reason
    is that a TLS protocol message can be up to 16 kbytes long (with TLSv1), and
    that an entire TLS protocol message must be transferred within the
    per-request deadline.
This feature is available in Postfix 3.7 and later. A weaker
    feature, called smtp_per_record_deadline, is available with Postfix
  2.9-3.6.
This feature is available in Postfix 3.7 and later.
How long the Postfix SMTP client pauses before sending
  ".<CR><LF>" in order to work around the PIX firewall
  "<CR><LF>.<CR><LF>" bug.
Choosing too short a time makes this workaround ineffective when
    sending large messages over slow network connections.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
Lookup tables, indexed by the remote SMTP server address, with per-destination
  workarounds for CISCO PIX firewall bugs. The table is not indexed by hostname
  for consistency with smtp_discard_ehlo_keyword_address_maps.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
This feature is available in Postfix 2.4 and later.
How long a message must be queued before the Postfix SMTP client turns on the
  PIX firewall "<CR><LF>.<CR><LF>" bug
  workaround for delivery through firewalls with "smtp fixup" mode
  turned on.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
By default, the workaround is turned off for mail that is queued
    for less than 500 seconds. In other words, the workaround is normally turned
    off for the first delivery attempt.
Specify 0 to enable the PIX firewall
    "<CR><LF>.<CR><LF>" bug workaround upon
    the first delivery attempt.
A list that specifies zero or more workarounds for CISCO PIX firewall bugs.
  These workarounds are implemented by the Postfix SMTP client. Workaround names
  are separated by comma or space, and are case insensitive. This parameter
  setting can be overruled with per-destination smtp_pix_workaround_maps
  settings.
  - delay_dotcrlf
- Insert a delay before sending ".<CR><LF>" after the
      end of the message content. The delay is subject to the
      smtp_pix_workaround_delay_time and smtp_pix_workaround_threshold_time
      parameter settings.
    
 
- disable_esmtp
- Disable all extended SMTP commands: send HELO instead of EHLO.
    
 
This feature is available in Postfix 2.4 and later. The default
    settings are backwards compatible with earlier Postfix versions.
The Postfix SMTP client time limit for sending the QUIT command, and for
  receiving the remote SMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
Quote addresses in Postfix SMTP client MAIL FROM and RCPT TO commands as
  required by RFC 5321. This includes putting quotes around an address localpart
  that ends in ".".
The default is to comply with RFC 5321. If you have to send mail
    to a broken SMTP server, configure a special SMTP client in master.cf:
  
/etc/postfix/master.cf:
    broken-smtp . . . smtp -o smtp_quote_rfc821_envelope=no
and route mail for the destination in question to the
    "broken-smtp" message delivery with a transport(5)
  table.
This feature is available in Postfix 2.1 and later.
Randomize the order of equal-preference MX host addresses. This is a performance
  feature of the Postfix SMTP client.
The Postfix SMTP client time limit for sending the SMTP RCPT TO command, and for
  receiving the remote SMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
A mechanism to transform replies from remote SMTP servers one line at a time.
  This is a last-resort tool to work around server replies that break
  interoperability with the Postfix SMTP client. Other uses involve fault
  injection to test Postfix's handling of invalid responses.
Notes:
  - In the case of a multi-line reply, the Postfix SMTP client uses the final
      reply line's numerical SMTP reply code and enhanced status code.
- The numerical SMTP reply code (XYZ) takes precedence over the enhanced
      status code (X.Y.Z). When the enhanced status code initial digit differs
      from the SMTP reply code initial digit, or when no enhanced status code is
      present, the Postfix SMTP client uses a generic enhanced status code
      (X.0.0) instead.
    
 
Specify the name of a "type:table" lookup table. The
    search string is a single SMTP reply line as received from the remote SMTP
    server, except that the trailing <CR><LF> are removed. When the
    lookup succeeds, the result replaces the single SMTP reply line.
Examples:
/etc/postfix/main.cf:
    smtp_reply_filter = pcre:/etc/postfix/reply_filter
/etc/postfix/reply_filter:
    # Transform garbage into "250-filler..." so that it looks like
    # one line from a multi-line reply. It does not matter what we
    # substitute here as long it has the right syntax.  The Postfix
    # SMTP client will use the final line's numerical SMTP reply
    # code and enhanced status code.
    !/^([2-5][0-9][0-9]($|[- ]))/ 250-filler for garbage
This feature is available in Postfix 2.7.
The Postfix SMTP client time limit for sending the RSET command, and for
  receiving the remote SMTP server response. The SMTP client sends RSET in order
  to finish a recipient address probe, or to verify that a cached session is
  still usable.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
An optional table to prevent repeated SASL authentication failures with the same
  remote SMTP server hostname, username and password. Each table (key, value)
  pair contains a server name, a username and password, and the full server
  response. This information is stored when a remote SMTP server rejects an
  authentication attempt with a 535 reply code. As long as the
  smtp_sasl_password_maps information does not change, and as long as the
  smtp_sasl_auth_cache_name information does not expire (see
  smtp_sasl_auth_cache_time) the Postfix SMTP client avoids SASL authentication
  attempts with the same server, username and password, and instead bounces or
  defers mail as controlled with the smtp_sasl_auth_soft_bounce configuration
  parameter.
Use a per-destination delivery concurrency of 1 (for example,
    "smtp_destination_concurrency_limit = 1",
    "relay_destination_concurrency_limit = 1", etc.), otherwise
    multiple delivery agents may experience a login failure at the same
  time.
The table must be accessed via the proxywrite service, i.e. the
    map name must start with "proxy:". The table should be stored
    under the directory specified with the data_directory parameter.
This feature uses cryptographic hashing to protect plain-text
    passwords, and requires that Postfix is compiled with TLS support.
Example:
smtp_sasl_auth_cache_name = proxy:btree:/var/db/postfix/sasl_auth_cache
This feature is available in Postfix 2.5 and later.
The maximal age of an smtp_sasl_auth_cache_name entry before it is removed.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is d (days).
This feature is available in Postfix 2.5 and later.
Enable SASL authentication in the Postfix SMTP client. By default, the Postfix
  SMTP client uses no authentication.
Example:
smtp_sasl_auth_enable = yes
When a remote SMTP server rejects a SASL authentication request with a 535 reply
  code, defer mail delivery instead of returning mail as undeliverable. The
  latter behavior was hard-coded prior to Postfix version 2.5.
Note: the setting "yes" overrides the global soft_bounce
    parameter, but the setting "no" does not.
Example:
# Default as of Postfix 2.5
smtp_sasl_auth_soft_bounce = yes
# The old hard-coded default
smtp_sasl_auth_soft_bounce = no
This feature is available in Postfix 2.5 and later.
If non-empty, a Postfix SMTP client filter for the remote SMTP server's list of
  offered SASL mechanisms. Different client and server implementations may
  support different mechanism lists; by default, the Postfix SMTP client will
  use the intersection of the two. smtp_sasl_mechanism_filter specifies an
  optional third mechanism list to intersect with.
Specify mechanism names, "/file/name" patterns or
    "type:table" lookup tables. The right-hand side result from
    "type:table" lookups is ignored. Specify "!pattern" to
    exclude a mechanism name from the list. The form "!/file/name" is
    supported only in Postfix version 2.4 and later.
This feature is available in Postfix 2.2 and later.
Examples:
smtp_sasl_mechanism_filter = plain, login
smtp_sasl_mechanism_filter = /etc/postfix/smtp_mechs
smtp_sasl_mechanism_filter = !gssapi, !login, static:rest
Optional Postfix SMTP client lookup tables with one username:password entry per
  sender, remote hostname or next-hop domain. Per-sender lookup is done only
  when sender-dependent authentication is enabled. If no username:password entry
  is found, then the Postfix SMTP client will not attempt to authenticate to the
  remote host.
The Postfix SMTP client opens the lookup table before going to
    chroot jail, so you can leave the password file in /etc/postfix.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
Implementation-specific information that the Postfix SMTP client passes through
  to the SASL plug-in implementation that is selected with
  smtp_sasl_type. Typically this specifies the name of a configuration
  file or rendezvous point.
This feature is available in Postfix 2.3 and later.
Postfix SMTP client SASL security options; as of Postfix 2.3 the list of
  available features depends on the SASL client implementation that is selected
  with smtp_sasl_type.
The following security features are defined for the cyrus
    client SASL implementation:
Specify zero or more of the following:
  - noplaintext
- Disallow methods that use plaintext passwords.
    
 
- noactive
- Disallow methods subject to active (non-dictionary) attack.
    
 
- nodictionary
- Disallow methods subject to passive (dictionary) attack.
    
 
- noanonymous
- Disallow methods that allow anonymous authentication.
    
 
- mutual_auth
- Only allow methods that provide mutual authentication (not available with
      SASL version 1).
    
 
Example:
smtp_sasl_security_options = noplaintext
The SASL authentication security options that the Postfix SMTP client uses for
  TLS encrypted SMTP sessions.
This feature is available in Postfix 2.2 and later.
The SASL authentication security options that the Postfix SMTP client uses for
  TLS encrypted SMTP sessions with a verified server certificate.
When mail is sent to the public MX host for the recipient's
    domain, server certificates are by default optional, and delivery proceeds
    even if certificate verification fails. For delivery via a submission
    service that requires SASL authentication, it may be appropriate to send
    plaintext passwords only when the connection to the server is strongly
    encrypted and the server identity is verified.
The smtp_sasl_tls_verified_security_options parameter makes it
    possible to only enable plaintext mechanisms when a secure connection to the
    server is available. Submission servers subject to this policy must either
    have verifiable certificates or offer suitable non-plaintext SASL
    mechanisms.
This feature is available in Postfix 2.6 and later.
The SASL plug-in type that the Postfix SMTP client should use for
  authentication. The available types are listed with the "postconf
  -A" command.
This feature is available in Postfix 2.3 and later.
Whether or not to append the "AUTH=<>" option to the MAIL FROM
  command in SASL-authenticated SMTP sessions. The default is not to send this,
  to avoid problems with broken remote SMTP servers. Before Postfix 2.9 the
  behavior is as if "smtp_send_dummy_mail_auth = yes".
This feature is available in Postfix 2.9 and later.
Send the non-standard XFORWARD command when the Postfix SMTP server EHLO
  response announces XFORWARD support.
This allows a Postfix SMTP delivery agent, used for injecting mail
    into a content filter, to forward the name, address, protocol and HELO name
    of the original client to the content filter and downstream queuing SMTP
    server. This can produce more useful logging than localhost[127.0.0.1]
  etc.
This feature is available in Postfix 2.1 and later.
Enable sender-dependent authentication in the Postfix SMTP client; this is
  available only with SASL authentication, and disables SMTP connection caching
  to ensure that mail from different senders will use the appropriate
  credentials.
This feature is available in Postfix 2.3 and later.
Skip SMTP servers that greet with a 4XX status code (go away, try again later).
By default, the Postfix SMTP client moves on the next mail
    exchanger. Specify "smtp_skip_4xx_greeting = no" if Postfix should
    defer delivery immediately.
This feature is available in Postfix 2.0 and earlier. Later
    Postfix versions always skip remote SMTP servers that greet with a 4XX
    status code.
Skip remote SMTP servers that greet with a 5XX status code.
By default, the Postfix SMTP client moves on the next mail
    exchanger. Specify "smtp_skip_5xx_greeting = no" if Postfix should
    bounce the mail immediately. Caution: the latter behavior appears to
    contradict RFC 2821.
Do not wait for the response to the SMTP QUIT command.
Time limit for Postfix SMTP client write and read operations during TLS startup
  and shutdown handshake procedures.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.2 and later.
The default TCP port that the Postfix SMTP client connects to. Specify a
  symbolic name (see services(5)) or a numeric port.
A file containing CA certificates of root CAs trusted to sign either remote SMTP
  server certificates or intermediate CA certificates. These are loaded into
  memory before the smtp(8) client enters the chroot jail. If the number
  of trusted roots is large, consider using smtp_tls_CApath instead, but note
  that the latter directory must be present in the chroot jail if the
  smtp(8) client is chrooted. This file may also be used to augment the
  client certificate trust chain, but it is best to include all the required
  certificates directly in $smtp_tls_cert_file (or, Postfix >= 3.4
  $smtp_tls_chain_files).
Specify "smtp_tls_CAfile = /path/to/system_CA_file" to
    use ONLY the system-supplied default Certification Authority
  certificates.
Specify "tls_append_default_CA = no" to prevent Postfix
    from appending the system-supplied default CAs and trusting third-party
    certificates.
Example:
smtp_tls_CAfile = /etc/postfix/CAcert.pem
This feature is available in Postfix 2.2 and later.
Directory with PEM format Certification Authority certificates that the Postfix
  SMTP client uses to verify a remote SMTP server certificate. Don't forget to
  create the necessary "hash" links with, for example,
  "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs".
To use this option in chroot mode, this directory (or a copy) must
    be inside the chroot jail.
Specify "smtp_tls_CApath = /path/to/system_CA_directory"
    to use ONLY the system-supplied default Certification Authority
    certificates.
Specify "tls_append_default_CA = no" to prevent Postfix
    from appending the system-supplied default CAs and trusting third-party
    certificates.
Example:
smtp_tls_CApath = /etc/postfix/certs
This feature is available in Postfix 2.2 and later.
Try to detect a mail hijacking attack based on a TLS protocol vulnerability
  (CVE-2009-3555), where an attacker prepends malicious HELO, MAIL, RCPT, DATA
  commands to a Postfix SMTP client TLS session. The attack would succeed with
  non-Postfix SMTP servers that reply to the malicious HELO, MAIL, RCPT, DATA
  commands after negotiating the Postfix SMTP client TLS session.
This feature is available in Postfix 2.7.
File with the Postfix SMTP client RSA certificate in PEM format. This file may
  also contain the Postfix SMTP client private RSA key, and these may be the
  same as the Postfix SMTP server RSA certificate and key file. With Postfix
  >= 3.4 the preferred way to configure client keys and certificates is via
  the "smtp_tls_chain_files" parameter.
Do not configure client certificates unless you must
    present client TLS certificates to one or more servers. Client certificates
    are not usually needed, and can cause problems in configurations that work
    well without them. The recommended setting is to let the defaults stand:
  
smtp_tls_cert_file =
smtp_tls_key_file =
smtp_tls_eccert_file =
smtp_tls_eckey_file =
# Obsolete DSA parameters
smtp_tls_dcert_file =
smtp_tls_dkey_file =
# Postfix >= 3.4 interface
smtp_tls_chain_files =
The best way to use the default settings is to comment out the
    above parameters in main.cf if present.
To enable remote SMTP servers to verify the Postfix SMTP client
    certificate, the issuing CA certificates must be made available to the
    server. You should include the required certificates in the client
    certificate file, the client certificate first, then the issuing CA(s)
    (bottom-up order).
Example: the certificate for "client.example.com" was
    issued by "intermediate CA" which itself has a certificate issued
    by "root CA". As the "root" super-user create the
    client.pem file with:
  
# umask 077
# cat client_key.pem client_cert.pem intermediate_CA.pem > chain.pem 
If you also want to verify remote SMTP server certificates issued
    by these CAs, you can add the CA certificates to the smtp_tls_CAfile, in
    which case it is not necessary to have them in the smtp_tls_cert_file,
    smtp_tls_dcert_file (obsolete) or smtp_tls_eccert_file.
A certificate supplied here must be usable as an SSL client
    certificate and hence pass the "openssl verify -purpose sslclient
    ..." test.
Example:
smtp_tls_cert_file = /etc/postfix/chain.pem
This feature is available in Postfix 2.2 and later.
List of one or more PEM files, each holding one or more private keys directly
  followed by a corresponding certificate chain. The file names are separated by
  commas and/or whitespace. This parameter obsoletes the legacy
  algorithm-specific key and certificate file settings. When this parameter is
  non-empty, the legacy parameters are ignored, and a warning is logged if any
  are also non-empty.
With the proliferation of multiple private key algorithms-which,
    as of OpenSSL 1.1.1, include DSA (obsolete), RSA, ECDSA, Ed25519 and
    Ed448-it is increasingly impractical to use separate parameters to configure
    the key and certificate chain for each algorithm. Therefore, Postfix now
    supports storing multiple keys and corresponding certificate chains in a
    single file or in a set of files.
Each key must appear immediately before the corresponding
    certificate, optionally followed by additional issuer certificates that
    complete the certificate chain for that key. When multiple files are
    specified, they are equivalent to a single file that is concatenated from
    those files in the given order. Thus, while a key must always precede its
    certificate and issuer chain, it can be in a separate file, so long as that
    file is listed immediately before the file that holds the corresponding
    certificate chain. Once all the files are concatenated, the sequence of PEM
    objects must be: key1, cert1, [chain1], key2, cert2, [chain2],
    ..., keyN, certN, [chainN].
Storing the private key in the same file as the corresponding
    certificate is more reliable. With the key and certificate in separate
    files, there is a chance that during key rollover a Postfix process might
    load a private key and certificate from separate files that don't match.
    Various operational errors may even result in a persistent broken
    configuration in which the certificate does not match the private key.
The file or files must contain at most one key of each type. If,
    for example, two or more RSA keys and corresponding chains are listed,
    depending on the version of OpenSSL either only the last one will be used or
    a configuration error may be detected. Note that while "Ed25519"
    and "Ed448" are considered separate algorithms, the various ECDSA
    curves (typically one of prime256v1, secp384r1 or secp521r1) are considered
    as different parameters of a single "ECDSA" algorithm, so it is
    not presently possible to configure keys for more than one ECDSA curve.
Example (separate files for each key and corresponding certificate
    chain):
  
/etc/postfix/main.cf:
    smtp_tls_chain_files =
        ${config_directory}/ed25519.pem,
        ${config_directory}/ed448.pem,
        ${config_directory}/rsa.pem
  
/etc/postfix/ed25519.pem:
    -----BEGIN PRIVATE KEY-----
    MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
    ...
    nC0egv51YPDWxEHom4QA
    -----END CERTIFICATE-----
  
/etc/postfix/ed448.pem:
    -----BEGIN PRIVATE KEY-----
    MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
    LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
    ...
    pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
    -----END CERTIFICATE-----
  
/etc/postfix/rsa.pem:
    -----BEGIN PRIVATE KEY-----
    MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
    ...
    ahQkZ3+krcaJvDSMgvu0tDc=
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
    ...
    Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
    -----END CERTIFICATE-----
Example (all keys and certificates in a single file):
  
/etc/postfix/main.cf:
    smtp_tls_chain_files = ${config_directory}/chains.pem
  
/etc/postfix/chains.pem:
    -----BEGIN PRIVATE KEY-----
    MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
    ...
    nC0egv51YPDWxEHom4QA
    -----END CERTIFICATE-----
    -----BEGIN PRIVATE KEY-----
    MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
    LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
    ...
    pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
    -----END CERTIFICATE-----
    -----BEGIN PRIVATE KEY-----
    MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
    ...
    ahQkZ3+krcaJvDSMgvu0tDc=
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
    ...
    Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
    -----END CERTIFICATE-----
This feature is available in Postfix 3.4 and later.
Obsolete Postfix < 2.3 control for the Postfix SMTP client TLS cipher list.
  As this feature applies to all TLS security levels, it is easy to create
  interoperability problems by choosing a non-default cipher list. Do not use a
  non-default TLS cipher list on hosts that deliver email to the public
  Internet: you will be unable to send email to servers that only support the
  ciphers you exclude. Using a restricted cipher list may be more appropriate
  for an internal MTA, where one can exert some control over the TLS software
  and settings of the peer servers.
Note: do not use "" quotes around the parameter
    value.
This feature is available in Postfix version 2.2. It is not used
    with Postfix 2.3 and later; use smtp_tls_mandatory_ciphers instead.
The minimum TLS cipher grade that the Postfix SMTP client will use with
  opportunistic TLS encryption. Cipher types listed in smtp_tls_exclude_ciphers
  are excluded from the base definition of the selected cipher grade. The
  default value is "medium" for Postfix releases after the middle of
  2015, "export" for older releases.
When TLS is mandatory the cipher grade is chosen via the
    smtp_tls_mandatory_ciphers configuration parameter, see there for syntax
    details. See smtp_tls_policy_maps for information on how to configure
    ciphers on a per-destination basis.
This feature is available in Postfix 2.6 and later. With earlier
    Postfix releases only the smtp_tls_mandatory_ciphers parameter is
    implemented, and opportunistic TLS always uses "export" or better
    (i.e. all) ciphers.
Try to make multiple deliveries per TLS-encrypted connection. This uses the
  tlsproxy(8) service to encrypt an SMTP connection, uses the
  scache(8) service to save that connection, and relies on hints from the
  qmgr(8) daemon.
See "Client-side TLS connection reuse" for background
    details.
This feature is available in Postfix 3.4 and later.
The TLS policy for MX hosts with "secure" TLSA records when the
  nexthop destination security level is dane, but the MX record was found
  via an "insecure" MX lookup. The choices are:
  - may
- The TLSA records will be ignored and TLS will be optional. If the MX host
      does not appear to support STARTTLS, or the STARTTLS handshake fails, mail
      may be sent in the clear.
    
 
- encrypt
- The TLSA records will signal a requirement to use TLS. While TLS
      encryption will be required, authentication will not be performed.
    
 
- dane
- The TLSA records will be used just as with "secure" MX records.
      TLS encryption will be required, and, if at least one of the TLSA records
      is "usable", authentication will be required. When
      authentication succeeds, it will be logged only as "Trusted",
      not "Verified", because the MX host name could have been forged.
    
 The default setting for Postfix >= 3.6 is "dane" with
      "smtp_tls_security_level = dane", otherwise "may".
      This behavior was backported to Postfix versions 3.5.9, 3.4.19, 3.3.16.
      3.2.21. With earlier Postfix versions the default setting was always
      "dane".
Though with "insecure" MX records an active attacker can
    compromise SMTP transport security by returning forged MX records, such
    attacks are "tamper-evident" since any forged MX hostnames will be
    recorded in the mail logs. Attackers who place a high value on staying
    hidden may be deterred from forging MX records.
This feature is available in Postfix 3.1 and later. The may
    policy is backwards-compatible with earlier Postfix versions.
File with the Postfix SMTP client DSA certificate in PEM format. This file may
  also contain the Postfix SMTP client private DSA key. The DSA algorithm is
  obsolete and should not be used.
See the discussion under smtp_tls_cert_file for more details.
Example:
smtp_tls_dcert_file = /etc/postfix/client-dsa.pem
This feature is available in Postfix 2.2 and later.
File with the Postfix SMTP client DSA private key in PEM format. This file may
  be combined with the Postfix SMTP client DSA certificate file specified with
  $smtp_tls_dcert_file. The DSA algorithm is obsolete and should not be used.
The private key must be accessible without a pass-phrase, i.e. it
    must not be encrypted. File permissions should grant read-only access to the
    system superuser account ("root"), and no access to anyone
  else.
This feature is available in Postfix 2.2 and later.
File with the Postfix SMTP client ECDSA certificate in PEM format. This file may
  also contain the Postfix SMTP client ECDSA private key. With Postfix >= 3.4
  the preferred way to configure client keys and certificates is via the
  "smtp_tls_chain_files" parameter.
See the discussion under smtp_tls_cert_file for more details.
Example:
smtp_tls_eccert_file = /etc/postfix/ecdsa-ccert.pem
This feature is available in Postfix 2.6 and later, when Postfix
    is compiled and linked with OpenSSL 1.0.0 or later.
File with the Postfix SMTP client ECDSA private key in PEM format. This file may
  be combined with the Postfix SMTP client ECDSA certificate file specified with
  $smtp_tls_eccert_file. With Postfix >= 3.4 the preferred way to configure
  client keys and certificates is via the "smtp_tls_chain_files"
  parameter.
The private key must be accessible without a pass-phrase, i.e. it
    must not be encrypted. File permissions should grant read-only access to the
    system superuser account ("root"), and no access to anyone
  else.
This feature is available in Postfix 2.6 and later, when Postfix
    is compiled and linked with OpenSSL 1.0.0 or later.
With mandatory TLS encryption, require that the remote SMTP server hostname
  matches the information in the remote SMTP server certificate. As of RFC 2487
  the requirements for hostname checking for MTA clients are not specified.
This option can be set to "no" to disable strict peer
    name checking. This setting has no effect on sessions that are controlled
    via the smtp_tls_per_site table.
Disabling the hostname verification can make sense in a closed
    environment where special CAs are created. If not used carefully, this
    option opens the danger of a "man-in-the-middle" attack (the
    CommonName of this attacker will be logged).
This feature is available in Postfix 2.2 and later. With Postfix
    2.3 and later use smtp_tls_security_level instead.
List of ciphers or cipher types to exclude from the Postfix SMTP client cipher
  list at all TLS security levels. This is not an OpenSSL cipherlist, it is a
  simple list separated by whitespace and/or commas. The elements are a single
  cipher, or one or more "+" separated cipher properties, in which
  case only ciphers matching all the properties are excluded.
Examples (some of these will cause problems):
  
smtp_tls_exclude_ciphers = aNULL
smtp_tls_exclude_ciphers = MD5, DES
smtp_tls_exclude_ciphers = DES+MD5
smtp_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
smtp_tls_exclude_ciphers = kEDH+aRSA
The first setting disables anonymous ciphers. The next setting
    disables ciphers that use the MD5 digest algorithm or the (single) DES
    encryption algorithm. The next setting disables ciphers that use MD5 and DES
    together. The next setting disables the two ciphers "AES256-SHA"
    and "DES-CBC3-MD5". The last setting disables ciphers that use
    "EDH" key exchange with RSA authentication.
This feature is available in Postfix 2.3 and later.
List of acceptable remote SMTP server certificate fingerprints for the
  "fingerprint" TLS security level (smtp_tls_security_level =
  fingerprint). At this security level, Certification Authorities are not used,
  and certificate expiration times are ignored. Instead, server certificates are
  verified directly via their certificate fingerprint or public key fingerprint
  (Postfix 2.9 and later). The fingerprint is a message digest of the server
  certificate (or public key). The digest algorithm is selected via the
  smtp_tls_fingerprint_digest parameter.
The colons between each pair of nibbles in the fingerprint value
    are optional (Postfix >= 3.6). These were required in earlier Postfix
    releases.
When an smtp_tls_policy_maps table entry specifies the
    "fingerprint" security level, any "match" attributes in
    that entry specify the list of valid fingerprints for the corresponding
    destination. Multiple fingerprints can be combined with a "|"
    delimiter in a single match attribute, or multiple match attributes can be
    employed.
Example: Certificate fingerprint verification with internal
    mailhub. Two matching fingerprints are listed. The relayhost may be multiple
    physical hosts behind a load-balancer, each with its own private/public key
    and self-signed certificate. Alternatively, a single relayhost may be in the
    process of switching from one set of private/public keys to another, and
    both keys are trusted just prior to the transition.
  
relayhost = [mailhub.example.com]
smtp_tls_security_level = fingerprint
smtp_tls_fingerprint_digest = sha256
smtp_tls_fingerprint_cert_match =
    cd:fc:d8:db:f8:c4:82:96:6c:...:28:71:e8:f5:8d:a5:0d:9b:d4:a6
    dd:5c:ef:f5:c3:bc:64:25:36:...:99:36:06:ce:40:ef:de:2e:ad:a4
Example: Certificate fingerprint verification with selected
    destinations. As in the example above, we show two matching
  fingerprints:
  
/etc/postfix/main.cf:
    smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
    smtp_tls_fingerprint_digest = sha256
  
/etc/postfix/tls_policy:
    example.com fingerprint
        match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76
        match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f
This feature is available in Postfix 2.5 and later.
The message digest algorithm used to construct remote SMTP server certificate
  fingerprints. At the "fingerprint" TLS security level
  (smtp_tls_security_level = fingerprint), the server certificate is
  verified by directly matching its certificate fingerprint or its public key
  fingerprint (Postfix 2.9 and later). The fingerprint is the message digest of
  the server certificate (or its public key) using the selected algorithm. With
  a digest algorithm resistant to "second pre-image" attacks, it is
  not feasible to create a new public key and a matching certificate (or
  public/private key-pair) that has the same fingerprint.
The default algorithm is sha256 with Postfix >= 3.6 and
    the compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
    the default algorithm is md5.
The best-practice algorithm is now sha256. Recent advances
    in hash function cryptanalysis have led to md5 and sha1 being deprecated in
    favor of sha256. However, as long as there are no known "second
    pre-image" attacks against the older algorithms, their use in this
    context, though not recommended, is still likely safe.
While additional digest algorithms are often available with
    OpenSSL's libcrypto, only those used by libssl in SSL cipher suites are
    available to Postfix. You'll likely find support for md5, sha1, sha256 and
    sha512.
To find the fingerprint of a specific certificate file, with a
    specific digest algorithm, run:
  
$ openssl x509 -noout -fingerprint -digest -in certfile.pem
The text to the right of the "=" sign is the desired
    fingerprint. For example:
  
$ openssl x509 -noout -fingerprint -sha256 -in cert.pem
SHA256 Fingerprint=D4:6A:AB:19:24:...:BB:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A
To extract the public key fingerprint from an X.509 certificate,
    you need to extract the public key from the certificate and compute the
    appropriate digest of its DER (ASN.1) encoding. With OpenSSL the
    "-pubkey" option of the "x509" command extracts the
    public key always in "PEM" format. We pipe the result to another
    OpenSSL command that converts the key to DER and then to the
    "dgst" command to compute the fingerprint.
The actual command to transform the key to DER format depends on
    the version of OpenSSL used. As of OpenSSL 1.0.0, the "pkey"
    command supports all key types.
  
# OpenSSL >= 1.0 with SHA-256 fingerprints.
$ openssl x509 -in cert.pem -noout -pubkey |
    openssl pkey -pubin -outform DER |
    openssl dgst -sha256 -c
(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:...:fc:09:1a:61:98:b5:bc:7c:60:58
The Postfix SMTP server and client log the peer (leaf) certificate
    fingerprint and the public key fingerprint when the TLS loglevel is 2 or
    higher.
This feature is available in Postfix 2.5 and later.
Lookup the associated DANE TLSA RRset even when a hostname is not an alias and
  its address records lie in an unsigned zone. This is unlikely to ever yield
  DNSSEC validated results, since child zones of unsigned zones are also
  unsigned in the absence of DLV or locally configured non-root trust-anchors.
  We anticipate that such mechanisms will not be used for just the
  "_tcp" subdomain of a host. Suppressing the TLSA RRset lookup
  reduces latency and avoids potential interoperability problems with
  nameservers for unsigned zones that are not prepared to handle the new TLSA
  RRset.
This feature is available in Postfix 2.11.
File with the Postfix SMTP client RSA private key in PEM format. This file may
  be combined with the Postfix SMTP client RSA certificate file specified with
  $smtp_tls_cert_file. With Postfix >= 3.4 the preferred way to configure
  client keys and certificates is via the "smtp_tls_chain_files"
  parameter.
The private key must be accessible without a pass-phrase, i.e. it
    must not be encrypted. File permissions should grant read-only access to the
    system superuser account ("root"), and no access to anyone
  else.
Example:
smtp_tls_key_file = $smtp_tls_cert_file
This feature is available in Postfix 2.2 and later.
Enable additional Postfix SMTP client logging of TLS activity. Each logging
  level also includes the information that is logged at a lower logging level.
  - 
  
- 0 Disable logging of TLS activity.
    
 
- 
  
- 1 Log only a summary message on TLS handshake completion - no logging of
      remote SMTP server certificate trust-chain verification errors if server
      certificate verification is not required. With Postfix 2.8 and earlier,
      log the summary message and unconditionally log trust-chain verification
      errors.
    
 
- 
  
- 2 Also log levels during TLS negotiation.
    
 
- 
  
- 3 Also log the hexadecimal and ASCII dump of the TLS negotiation process.
    
 
- 
  
- 4 Also log the hexadecimal and ASCII dump of complete transmission after
      STARTTLS.
    
 
Do not use "smtp_tls_loglevel = 2" or higher except in
    case of problems. Use of loglevel 4 is strongly discouraged.
This feature is available in Postfix 2.2 and later.
The minimum TLS cipher grade that the Postfix SMTP client will use with
  mandatory TLS encryption. The default value "medium" is suitable for
  most destinations with which you may want to enforce TLS, and is beyond the
  reach of today's cryptanalytic methods. See smtp_tls_policy_maps for
  information on how to configure ciphers on a per-destination basis.
The following cipher grades are supported:
  - high
- Enable only "HIGH" grade OpenSSL ciphers. This setting may be
      appropriate when all mandatory TLS destinations (e.g. when all mail is
      routed to a suitably capable relayhost) support at least one
      "HIGH" grade cipher. The underlying cipherlist is specified via
      the tls_high_cipherlist configuration parameter, which you are strongly
      encouraged not to change.
    
 
- medium
- Enable "MEDIUM" grade or better OpenSSL ciphers. The underlying
      cipherlist is specified via the tls_medium_cipherlist configuration
      parameter, which you are strongly encouraged not to change.
    
 
- null
- Enable only the "NULL" OpenSSL ciphers, these provide
      authentication without encryption. This setting is only appropriate in the
      rare case that all servers are prepared to use NULL ciphers (not normally
      enabled in TLS servers). A plausible use-case is an LMTP server listening
      on a UNIX-domain socket that is configured to support "NULL"
      ciphers. The underlying cipherlist is specified via the
      tls_null_cipherlist configuration parameter, which you are strongly
      encouraged not to change.
    
 
- low
- Enable "LOW" grade or stronger OpenSSL ciphers. In Postfix >=
      3.8 this cipher grade is always identical to "medium". Recent
      versions of OpenSSL do not support any "LOW" grade ciphers. In
      earlier Postfix releases the underlying cipherlist was specified via the
      tls_low_cipherlist configuration parameter, which you are strongly
      encouraged not to change. This obsolete cipher grade SHOULD NOT be used.
    
 
- export
- Enable "EXPORT" grade or stronger OpenSSL ciphers. In Postfix
      >= 3.8 this cipher grade is always identical to "medium".
      Recent versions of OpenSSL do not support any "EXPORT" grade
      ciphers. In earlier Postfix releases the underlying cipherlist was
      specified via the tls_export_cipherlist configuration parameter, which you
      are strongly encouraged not to change. This obsolete cipher grade SHOULD
      NOT be used.
    
 
The underlying cipherlists for grades other than "null"
    include anonymous ciphers, but these are automatically filtered out if the
    Postfix SMTP client is configured to verify server certificates. You are
    very unlikely to need to take any steps to exclude anonymous ciphers, they
    are excluded automatically as necessary. If you must exclude anonymous
    ciphers at the "may" or "encrypt" security levels, when
    the Postfix SMTP client does not need or use peer certificates, set
    "smtp_tls_exclude_ciphers = aNULL". To exclude anonymous ciphers
    only when TLS is enforced, set "smtp_tls_mandatory_exclude_ciphers =
    aNULL".
This feature is available in Postfix 2.3 and later.
Additional list of ciphers or cipher types to exclude from the Postfix SMTP
  client cipher list at mandatory TLS security levels. This list works in
  addition to the exclusions listed with smtp_tls_exclude_ciphers (see there for
  syntax details).
Starting with Postfix 2.6, the mandatory cipher exclusions can be
    specified on a per-destination basis via the TLS policy "exclude"
    attribute. See smtp_tls_policy_maps for notes and examples.
This feature is available in Postfix 2.3 and later.
TLS protocols that the Postfix SMTP client will use with mandatory TLS
  encryption. In main.cf the values are separated by whitespace, commas or
  colons. In the policy table "protocols" attribute (see
  smtp_tls_policy_maps) the only valid separator is colon. An empty value means
  allow all protocols.
The valid protocol names (see SSL_get_version(3)) are
    "SSLv2", "SSLv3", "TLSv1",
    "TLSv1.1", "TLSv1.2" and "TLSv1.3". Starting
    with Postfix 3.6, the default value is ">=TLSv1", which sets
    TLS 1.0 as the lowest supported TLS protocol version (see below). Older
    releases use the "!" exclusion syntax, also described below.
As of Postfix 3.6, the preferred way to limit the range of
    acceptable protocols is to set a lowest acceptable TLS protocol version
    and/or a highest acceptable TLS protocol version. To set the lower bound
    include an element of the form: ">=version" where
    version is a either one of the TLS protocol names listed above, or a
    hexadecimal number corresponding to the desired TLS protocol version (0301
    for TLS 1.0, 0302 for TLS 1.1, etc.). For the upper bound, use
    "<=version". There must be no whitespace between the
    ">=" or "<=" symbols and the protocol name or
    number.
Hexadecimal protocol numbers make it possible to specify protocol
    bounds for TLS versions that are known to OpenSSL, but might not be known to
    Postfix. They cannot be used with the legacy exclusion syntax. Leading
    "0" or "0x" prefixes are supported, but not required.
    Therefore, "301", "0301", "0x301" and
    "0x0301" are all equivalent to "TLSv1". Hexadecimal
    versions unknown to OpenSSL will fail to set the upper or lower bound, and a
    warning will be logged. Hexadecimal versions should only be used when
    Postfix is linked with some future version of OpenSSL that supports TLS 1.4
    or later, but Postfix does not yet support a symbolic name for that protocol
    version.
Hexadecimal example (Postfix >= 3.6):
  
# Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtp_tls_mandatory_protocols = >=TLSv1.2, <=0305
# Allow only TLS 1.2 and up:
smtp_tls_mandatory_protocols = >=0x0303
With Postfix < 3.6 there is no support for a minimum or maximum
    version, and the protocol range is configured via protocol exclusions. To
    require at least TLS 1.0, set "smtp_tls_mandatory_protocols = !SSLv2,
    !SSLv3". Listing the protocols to include, rather than the protocols to
    exclude, is supported, but not recommended. The exclusion syntax more
    accurately matches the underlying OpenSSL interface.
When using the exclusion syntax, take care to ensure that the
    range of protocols supported by the Postfix SMTP client is contiguous. When
    a protocol version is enabled, disabling any higher version implicitly
    disables all versions above that higher version. Thus, for example:
  
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1.1
also disables any protocol versions higher than TLSv1.1 leaving
    only "TLSv1" enabled.
Support for "TLSv1.3" was introduced in OpenSSL 1.1.1.
    Disabling this protocol via "!TLSv1.3" is supported since Postfix
    3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).
While the vast majority of SMTP servers with DANE TLSA records now
    support at least TLS 1.2, a few still only support TLS 1.0. If you use
    "dane" or "dane-only" it is best not to disable TLSv1,
    except perhaps via the policy table for destinations which you are sure will
    support "TLSv1.2".
See the documentation of the smtp_tls_policy_maps parameter and
    TLS_README for more information about security levels.
Example:
# Preferred syntax with Postfix >= 3.6:
smtp_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
# Legacy syntax:
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
This feature is available in Postfix 2.3 and later.
Log the hostname of a remote SMTP server that offers STARTTLS, when TLS is not
  already enabled for that server.
The logfile record looks like:
postfix/smtp[pid]:  Host offered STARTTLS: [name.of.host]
This feature is available in Postfix 2.2 and later.
Optional lookup tables with the Postfix SMTP client TLS usage policy by next-hop
  destination and by remote SMTP server hostname. When both lookups succeed, the
  more specific per-site policy (NONE, MUST, etc.) overrides the less specific
  one (MAY), and the more secure per-site policy (MUST, etc.) overrides the less
  secure one (NONE). With Postfix 2.3 and later smtp_tls_per_site is strongly
  discouraged: use smtp_tls_policy_maps instead.
Use of the bare hostname as the per-site table lookup key is
    discouraged. Always use the full destination nexthop (enclosed in [] with a
    possible ":port" suffix). A recipient domain or MX-enabled
    transport next-hop with no port suffix may look like a bare hostname, but is
    still a suitable destination.
Specify a next-hop destination or server hostname on the left-hand
    side; no wildcards are allowed. The next-hop destination is either the
    recipient domain, or the destination specified with a transport(5)
    table, the relayhost parameter, or the relay_transport parameter. On the
    right hand side specify one of the following keywords:
  - NONE
- Don't use TLS at all. This overrides a less specific MAY lookup
      result from the alternate host or next-hop lookup key, and overrides the
      global smtp_use_tls, smtp_enforce_tls, and smtp_tls_enforce_peername
      settings.
    
 
- MAY
- Try to use TLS if the server announces support, otherwise use an
      unencrypted connection. This has less precedence than a more specific
      result (including NONE) from the alternate host or next-hop lookup
      key, and has less precedence than the more specific global
      "smtp_enforce_tls = yes" or "smtp_tls_enforce_peername =
      yes".
    
 
- MUST_NOPEERMATCH
- Require TLS encryption, but do not require that the remote SMTP server
      hostname matches the information in the remote SMTP server certificate, or
      that the server certificate was issued by a trusted CA. This overrides a
      less secure NONE or a less specific MAY lookup result from
      the alternate host or next-hop lookup key, and overrides the global
      smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername settings.
    
 
- MUST
- Require TLS encryption, require that the remote SMTP server hostname
      matches the information in the remote SMTP server certificate, and require
      that the remote SMTP server certificate was issued by a trusted CA. This
      overrides a less secure NONE or MUST_NOPEERMATCH or a less
      specific MAY lookup result from the alternate host or next-hop
      lookup key, and overrides the global smtp_use_tls, smtp_enforce_tls and
      smtp_tls_enforce_peername settings.
    
 
The above keywords correspond to the "none",
    "may", "encrypt" and "verify" security levels
    for the new smtp_tls_security_level parameter introduced in Postfix 2.3.
    Starting with Postfix 2.3, and independently of how the policy is specified,
    the smtp_tls_mandatory_ciphers and smtp_tls_mandatory_protocols parameters
    apply when TLS encryption is mandatory. Connections for which encryption is
    optional typically enable all "export" grade and better ciphers
    (see smtp_tls_ciphers and smtp_tls_protocols).
As long as no secure DNS lookup mechanism is available, false
    hostnames in MX or CNAME responses can change the server hostname that
    Postfix uses for TLS policy lookup and server certificate verification. Even
    with a perfect match between the server hostname and the server certificate,
    there is no guarantee that Postfix is connected to the right server. See
    TLS_README (Closing a DNS loophole with obsolete per-site TLS policies) for
    a possible work-around.
This feature is available in Postfix 2.2 and later. With Postfix
    2.3 and later use smtp_tls_policy_maps instead.
Optional lookup tables with the Postfix SMTP client TLS security policy by
  next-hop destination; when a non-empty value is specified, this overrides the
  obsolete smtp_tls_per_site parameter. See TLS_README for a more detailed
  discussion of TLS security levels.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
The TLS policy table is indexed by the full next-hop destination,
    which is either the recipient domain, or the verbatim next-hop specified in
    the transport table, $local_transport, $virtual_transport, $relay_transport
    or $default_transport. This includes any enclosing square brackets and any
    non-default destination server port suffix. The LMTP socket type prefix
    (inet: or unix:) is not included in the lookup key.
Only the next-hop domain, or $myhostname with LMTP over
    UNIX-domain sockets, is used as the nexthop name for certificate
    verification. The port and any enclosing square brackets are used in the
    table lookup key, but are not used for server name verification.
When the lookup key is a domain name without enclosing square
    brackets or any :port suffix (typically the recipient domain), and
    the full domain is not found in the table, just as with the
    transport(5) table, the parent domain starting with a leading
    "." is matched recursively. This allows one to specify a security
    policy for a recipient domain and all its sub-domains.
The lookup result is a security level, followed by an optional
    list of whitespace and/or comma separated name=value attributes that
    override related main.cf settings. The TLS security levels in order of
    increasing security are:
  - none
- No TLS. No additional attributes are supported at this level.
    
 
- may
- Opportunistic TLS. Since sending in the clear is acceptable, demanding
      stronger than default TLS security merely reduces interoperability. The
      optional "ciphers", "exclude", and
      "protocols" attributes (available for opportunistic TLS with
      Postfix >= 2.6) and "connection_reuse" attribute (Postfix
      >= 3.4) override the "smtp_tls_ciphers",
      "smtp_tls_exclude_ciphers", "smtp_tls_protocols", and
      "smtp_tls_connection_reuse" configuration parameters. In the
      policy table, multiple ciphers, protocols or excluded ciphers must be
      separated by colons, as attribute values may not contain whitespace or
      commas. When opportunistic TLS handshakes fail, Postfix retries the
      connection with TLS disabled. This allows mail delivery to sites with
      non-interoperable TLS implementations.
    
 
- encrypt
- Mandatory TLS encryption. At this level and higher, the optional
      "protocols" attribute overrides the main.cf
      smtp_tls_mandatory_protocols parameter, the optional "ciphers"
      attribute overrides the main.cf smtp_tls_mandatory_ciphers parameter, the
      optional "exclude" attribute (Postfix >= 2.6) overrides the
      main.cf smtp_tls_mandatory_exclude_ciphers parameter, and the optional
      "connection_reuse" attribute (Postfix >= 3.4) overrides the
      main.cf smtp_tls_connection_reuse parameter. In the policy table, multiple
      ciphers, protocols or excluded ciphers must be separated by colons, as
      attribute values may not contain whitespace or commas.
    
 
- dane
- Opportunistic DANE TLS. The TLS policy for the destination is obtained via
      TLSA records in DNSSEC. If no TLSA records are found, the effective
      security level used is may. If TLSA records are found, but none are
      usable, the effective security level is encrypt. When usable TLSA records
      are obtained for the remote SMTP server, the server certificate must match
      the TLSA records. RFC 7672 (DANE) TLS authentication and DNSSEC support is
      available with Postfix 2.11 and later. The optional
      "connection_reuse" attribute (Postfix >= 3.4) overrides the
      main.cf smtp_tls_connection_reuse parameter. When the effective security
      level used is may, the optional "ciphers", "exclude",
      and "protocols" attributes (Postfix >= 2.6) override the
      "smtp_tls_ciphers", "smtp_tls_exclude_ciphers", and
      "smtp_tls_protocols" configuration parameters. When the
      effective security level used is encrypt, the optional
      "ciphers", "exclude", and "protocols"
      attributes (Postfix >= 2.6) override the
      "smtp_tls_mandatory_ciphers",
      "smtp_tls_mandatory_exclude_ciphers", and
      "smtp_tls_mandatory_protocols" configuration parameters.
    
 
- dane-only
- Mandatory DANE TLS. The TLS policy for the destination is obtained via
      TLSA records in DNSSEC. If no TLSA records are found, or none are usable,
      no connection is made to the server. When usable TLSA records are obtained
      for the remote SMTP server, the server certificate must match the TLSA
      records. RFC 7672 (DANE) TLS authentication and DNSSEC support is
      available with Postfix 2.11 and later. The optional "ciphers",
      "exclude", and "protocols" attributes (Postfix >=
      2.6) override the "smtp_tls_mandatory_ciphers",
      "smtp_tls_mandatory_exclude_ciphers", and
      "smtp_tls_mandatory_protocols" configuration parameters. The
      optional "connection_reuse" attribute (Postfix >= 3.4)
      overrides the main.cf smtp_tls_connection_reuse parameter.
    
 
- fingerprint
- Certificate fingerprint verification. Available with Postfix 2.5 and
      later. At this security level, there are no trusted Certification
      Authorities. The certificate trust chain, expiration date, ... are not
      checked. Instead, the optional "match" attribute, or else the
      main.cf smtp_tls_fingerprint_cert_match parameter, lists the
      certificate fingerprints or the public key fingerprint (Postfix 2.9 and
      later) of the valid server certificate. The digest algorithm used to
      calculate the fingerprint is selected by the
      smtp_tls_fingerprint_digest parameter. Multiple fingerprints can be
      combined with a "|" delimiter in a single match attribute, or
      multiple match attributes can be employed. The ":" character is
      not used as a delimiter as it occurs between each pair of fingerprint
      (hexadecimal) digits. The optional "ciphers",
      "exclude", and "protocols" attributes (Postfix >=
      2.6) override the "smtp_tls_mandatory_ciphers",
      "smtp_tls_mandatory_exclude_ciphers", and
      "smtp_tls_mandatory_protocols" configuration parameters. The
      optional "connection_reuse" attribute (Postfix >= 3.4)
      overrides the main.cf smtp_tls_connection_reuse parameter.
    
 
- verify
- Mandatory TLS verification. At this security level, DNS MX lookups are
      trusted to be secure enough, and the name verified in the server
      certificate is usually obtained indirectly via unauthenticated DNS MX
      lookups. The optional "match" attribute overrides the main.cf
      smtp_tls_verify_cert_match parameter. In the policy table, multiple match
      patterns and strategies must be separated by colons. In practice explicit
      control over matching is more common with the "secure" policy,
      described below. The optional "ciphers", "exclude",
      and "protocols" attributes (Postfix >= 2.6) override the
      "smtp_tls_mandatory_ciphers",
      "smtp_tls_mandatory_exclude_ciphers", and
      "smtp_tls_mandatory_protocols" configuration parameters. The
      optional "connection_reuse" attribute (Postfix >= 3.4)
      overrides the main.cf smtp_tls_connection_reuse parameter.
    
 
- secure
- Secure-channel TLS. At this security level, DNS MX lookups, though
      potentially used to determine the candidate next-hop gateway IP addresses,
      are not trusted to be secure enough for TLS peername verification.
      Instead, the default name verified in the server certificate is obtained
      directly from the next-hop, or is explicitly specified via the optional
      "match" attribute which overrides the main.cf
      smtp_tls_secure_cert_match parameter. In the policy table, multiple match
      patterns and strategies must be separated by colons. The match attribute
      is most useful when multiple domains are supported by a common server: the
      policy entries for additional domains specify matching rules for the
      primary domain certificate. While transport table overrides that route the
      secondary domains to the primary nexthop also allow secure verification,
      they risk delivery to the wrong destination when domains change hands or
      are re-assigned to new gateways. With the "match" attribute
      approach, routing is not perturbed, and mail is deferred if verification
      of a new MX host fails. The optional "ciphers",
      "exclude", and "protocols" attributes (Postfix >=
      2.6) override the "smtp_tls_mandatory_ciphers",
      "smtp_tls_mandatory_exclude_ciphers", and
      "smtp_tls_mandatory_protocols" configuration parameters. The
      optional "connection_reuse" attribute (Postfix >= 3.4)
      overrides the main.cf smtp_tls_connection_reuse parameter.
    
 
Example:
/etc/postfix/main.cf:
    smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
    # Postfix 2.5 and later.
    #
    # The default digest is sha256 with Postfix >= 3.6 and
    # compatibility level >= 3.
    #
    smtp_tls_fingerprint_digest = sha256
/etc/postfix/tls_policy:
    example.edu                 none
    example.mil                 may
    example.gov                 encrypt protocols=TLSv1
    example.com                 verify ciphers=high
    example.net                 secure
    .example.net                secure match=.example.net:example.net
    [mail.example.org]:587      secure match=nexthop
    # Postfix 2.5 and later
    [thumb.example.org]          fingerprint
        match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f
        match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76
Note: The "hostname" strategy if listed in a
    non-default setting of smtp_tls_secure_cert_match or in the
    "match" attribute in the policy table can render the
    "secure" level vulnerable to DNS forgery. Do not use the
    "hostname" strategy for secure-channel configurations in
    environments where DNS security is not assured.
This feature is available in Postfix 2.3 and later.
TLS protocols that the Postfix SMTP client will use with opportunistic TLS
  encryption. In main.cf the values are separated by whitespace, commas or
  colons. In the policy table "protocols" attribute (see
  smtp_tls_policy_maps) the only valid separator is colon. An empty value means
  allow all protocols.
The valid protocol names (see SSL_get_version(3)) are
    "SSLv2", "SSLv3", "TLSv1",
    "TLSv1.1", "TLSv1.2" and "TLSv1.3". Starting
    with Postfix 3.6, the default value is ">=TLSv1", which sets
    TLS 1.0 as the lowest supported TLS protocol version (see below). Older
    releases use the "!" exclusion syntax, also described below.
As of Postfix 3.6, the preferred way to limit the range of
    acceptable protocols is to set the lowest acceptable TLS protocol version
    and/or the highest acceptable TLS protocol version. To set the lower bound
    include an element of the form: ">=version" where
    version is either one of the TLS protocol names listed above, or a
    hexadecimal number corresponding to the desired TLS protocol version (0301
    for TLS 1.0, 0302 for TLS 1.1, etc.). For the upper bound, use
    "<=version". There must be no whitespace between the
    ">=" or "<=" symbols and the protocol name or
    number.
Hexadecimal protocol numbers make it possible to specify protocol
    bounds for TLS versions that are known to OpenSSL, but might not be known to
    Postfix. They cannot be used with the legacy exclusion syntax. Leading
    "0" or "0x" prefixes are supported, but not required.
    Therefore, "301", "0301", "0x301" and
    "0x0301" are all equivalent to "TLSv1". Hexadecimal
    versions unknown to OpenSSL will fail to set the upper or lower bound, and a
    warning will be logged. Hexadecimal versions should only be used when
    Postfix is linked with some future version of OpenSSL that supports TLS 1.4
    or later, but Postfix does not yet support a symbolic name for that protocol
    version.
Hexadecimal example (Postfix >= 3.6):
  
# Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtp_tls_protocols = >=TLSv1, <=0305
# Allow only TLS 1.0 and up:
smtp_tls_protocols = >=0x0301
With Postfix < 3.6 there is no support for a minimum or maximum
    version, and the protocol range is configured via protocol exclusions. To
    require at least TLS 1.0, set "smtp_tls_protocols = !SSLv2,
    !SSLv3". Listing the protocols to include, rather than protocols to
    exclude, is supported, but not recommended. The exclusion form more
    accurately matches the underlying OpenSSL interface.
When using the exclusion syntax, take care to ensure that the
    range of protocols advertised by an SSL/TLS client is contiguous. When a
    protocol version is enabled, disabling any higher version implicitly
    disables all versions above that higher version. Thus, for example:
  
smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1.1
also disables any protocols version higher than TLSv1.1 leaving only
  "TLSv1" enabled.
Support for "TLSv1.3" was introduced in OpenSSL 1.1.1.
    Disabling this protocol via "!TLSv1.3" is supported since Postfix
    3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).
Example:
# Preferred syntax with Postfix >= 3.6:
smtp_tls_protocols = >=TLSv1, <=TLSv1.3
# Legacy syntax:
smtp_tls_protocols = !SSLv2, !SSLv3
This feature is available in Postfix 2.6 and later.
The verification depth for remote SMTP server certificates. A depth of 1 is
  sufficient if the issuing CA is listed in a local CA file.
The default verification depth is 9 (the OpenSSL default) for
    compatibility with earlier Postfix behavior. Prior to Postfix 2.5, the
    default value was 5, but the limit was not actually enforced. If you have
    set this to a lower non-default value, certificates with longer trust chains
    may now fail to verify. Certificate chains with 1 or 2 CAs are common,
    deeper chains are more rare and any number between 5 and 9 should suffice in
    practice. You can choose a lower number if, for example, you trust
    certificates directly signed by an issuing CA but not any CAs it delegates
    to.
This feature is available in Postfix 2.2 and later.
How the Postfix SMTP client verifies the server certificate peername for the
  "secure" TLS security level. In a "secure" TLS policy
  table ($smtp_tls_policy_maps) entry the optional "match" attribute
  overrides this main.cf setting.
This parameter specifies one or more patterns or strategies
    separated by commas, whitespace or colons. In the policy table the only
    valid separator is the colon character.
For a description of the pattern and strategy syntax see the
    smtp_tls_verify_cert_match parameter. The "hostname" strategy
    should be avoided in this context, as in the absence of a secure global DNS,
    using the results of MX lookups in certificate verification is not immune to
    active (man-in-the-middle) attacks on DNS.
Sample main.cf setting:
  
smtp_tls_secure_cert_match = nexthop
Sample policy table override:
  
example.net     secure match=example.com:.example.com
.example.net    secure match=example.com:.example.com
This feature is available in Postfix 2.3 and later.
The default SMTP TLS security level for the Postfix SMTP client. When a
  non-empty value is specified, this overrides the obsolete parameters
  smtp_use_tls, smtp_enforce_tls, and smtp_tls_enforce_peername; when no value
  is specified for smtp_tls_enforce_peername or the obsolete parameters, the
  default SMTP TLS security level is none.
Specify one of the following security levels:
  - none
- No TLS. TLS will not be used unless enabled for specific destinations via
      smtp_tls_policy_maps.
    
 
- may
- Opportunistic TLS. Use TLS if this is supported by the remote SMTP server,
      otherwise use plaintext. Since sending in the clear is acceptable,
      demanding stronger than default TLS security merely reduces
      interoperability. The "smtp_tls_ciphers" and
      "smtp_tls_protocols" (Postfix >= 2.6) configuration
      parameters provide control over the protocols and cipher grade used with
      opportunistic TLS. With earlier releases the opportunistic TLS cipher
      grade is always "export" and no protocols are disabled. When TLS
      handshakes fail, the connection is retried with TLS disabled. This allows
      mail delivery to sites with non-interoperable TLS implementations.
    
 
- encrypt
- Mandatory TLS encryption. Since a minimum level of security is intended,
      it is reasonable to be specific about sufficiently secure protocol
      versions and ciphers. At this security level and higher, the main.cf
      parameters smtp_tls_mandatory_protocols and smtp_tls_mandatory_ciphers
      specify the TLS protocols and minimum cipher grade which the administrator
      considers secure enough for mandatory encrypted sessions. This security
      level is not an appropriate default for systems delivering mail to the
      Internet.
    
 
- dane
- Opportunistic DANE TLS. At this security level, the TLS policy for the
      destination is obtained via DNSSEC. For TLSA policy to be in effect, the
      destination domain's containing DNS zone must be signed and the Postfix
      SMTP client's operating system must be configured to send its DNS queries
      to a recursive DNS nameserver that is able to validate the signed records.
      Each MX host's DNS zone should also be signed, and should publish DANE
      TLSA (RFC 7672) records that specify how that MX host's TLS certificate is
      to be verified. TLSA records do not preempt the normal SMTP MX host
      selection algorithm, if some MX hosts support TLSA and others do not, TLS
      security will vary from delivery to delivery. It is up to the domain owner
      to configure their MX hosts and their DNS sensibly. To configure the
      Postfix SMTP client for DNSSEC lookups see the documentation for the
      smtp_dns_support_level main.cf parameter. When DNSSEC-validated TLSA
      records are not found the effective tls security level is "may".
      When TLSA records are found, but are all unusable the effective security
      level is "encrypt". For purposes of protocol and cipher
      selection, the "dane" security level is treated like a
      "mandatory" TLS security level, and weak ciphers and protocols
      are disabled. Since DANE authenticates server certificates the
      "aNULL" cipher-suites are transparently excluded at this level,
      no need to configure this manually. RFC 7672 (DANE) TLS authentication is
      available with Postfix 2.11 and later.
    
 
- dane-only
- Mandatory DANE TLS. This is just like "dane" above, but DANE
      TLSA authentication is required. There is no fallback to "may"
      or "encrypt" when TLSA records are missing or unusable. RFC 7672
      (DANE) TLS authentication is available with Postfix 2.11 and later.
    
 
- fingerprint
- Certificate fingerprint verification. At this security level, there are no
      trusted Certification Authorities. The certificate trust chain, expiration
      date, etc., are not checked. Instead, the
      smtp_tls_fingerprint_cert_match parameter lists the certificate
      fingerprint or public key fingerprint (Postfix 2.9 and later) of the valid
      server certificate. The digest algorithm used to calculate the fingerprint
      is selected by the smtp_tls_fingerprint_digest parameter. Available
      with Postfix 2.5 and later.
    
 
- verify
- Mandatory TLS verification. At this security level, DNS MX lookups are
      trusted to be secure enough, and the name verified in the server
      certificate is usually obtained indirectly via unauthenticated DNS MX
      lookups. The smtp_tls_verify_cert_match parameter controls how the server
      name is verified. In practice explicit control over matching is more
      common at the "secure" level, described below. This security
      level is not an appropriate default for systems delivering mail to the
      Internet.
    
 
- secure
- Secure-channel TLS. At this security level, DNS MX lookups, though
      potentially used to determine the candidate next-hop gateway IP addresses,
      are not trusted to be secure enough for TLS peername verification.
      Instead, the default name verified in the server certificate is obtained
      from the next-hop domain as specified in the smtp_tls_secure_cert_match
      configuration parameter. The default matching rule is that a server
      certificate matches when its name is equal to or is a sub-domain of the
      nexthop domain. This security level is not an appropriate default for
      systems delivering mail to the Internet.
    
 
Examples:
# No TLS. Formerly: smtp_use_tls=no and smtp_enforce_tls=no.
smtp_tls_security_level = none
# Opportunistic TLS.
smtp_tls_security_level = may
# Do not tweak opportunistic ciphers or protocols unless it is essential
# to do so (if a security vulnerability is found in the SSL library that
# can be mitigated by disabling a particular protocol or raising the
# cipher grade).
smtp_tls_ciphers = medium
smtp_tls_protocols = >=TLSv1
# Legacy (Postfix < 3.6) syntax:
smtp_tls_protocols = !SSLv2, !SSLv3
# Mandatory (high-grade) TLS encryption.
smtp_tls_security_level = encrypt
smtp_tls_mandatory_ciphers = high
# Authenticated TLS 1.2 or better matching the nexthop domain or a
# subdomain.
smtp_tls_security_level = secure
smtp_tls_mandatory_ciphers = high
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_secure_cert_match = nexthop, dot-nexthop
# Certificate fingerprint verification (Postfix >= 2.5).
# The CA-less "fingerprint" security level only scales to a limited
# number of destinations. As a global default rather than a per-site
# setting, this is practical only when mail for all recipients is sent
# to a central mail hub.
relayhost = [mailhub.example.com]
smtp_tls_security_level = fingerprint
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_mandatory_ciphers = high
smtp_tls_fingerprint_cert_match =
    3D:95:34:51:...:40:99:C0:C1
    EC:3B:2D:B0:...:A3:9D:72:F6
This feature is available in Postfix 2.3 and later.
Optional name to send to the remote SMTP server in the TLS Server Name
  Indication (SNI) extension. The SNI extension is always on when DANE is used
  to authenticate the server, and in that case the SNI name sent is the one
  required by RFC7672 and this parameter is ignored.
Some SMTP servers use the received SNI name to select an
    appropriate certificate chain to present to the client. While this may
    improve interoperability with such servers, it may reduce interoperability
    with other servers that choose to abort the connection when they don't have
    a certificate chain configured for the requested name. Such servers should
    select a default certificate chain and continue the handshake, but some may
    not. Therefore, absent DANE, no SNI name is sent by default.
The SNI name must be either a valid DNS hostname, or else one of
    the special values hostname or nexthop, which select either
    the remote hostname or the nexthop domain respectively. DNS names for SNI
    must be in A-label (punycode) form. Invalid DNS names log a configuration
    error warning and mail delivery is deferred.
Except when using a relayhost to forward all email, the only
    sensible non-empty main.cf setting for this parameter is hostname.
    Other non-empty values are only practical on a per-destination basis via the
    servername attribute of the Postfix TLS policy table. When in doubt,
    leave this parameter empty, and configure per-destination SNI as needed.
This feature is available in Postfix 3.4 and later.
Name of the file containing the optional Postfix SMTP client TLS session cache.
  Specify a database type that supports enumeration, such as btree or
  sdbm; there is no need to support concurrent access. The file is
  created if it does not exist. The smtp(8) daemon does not use this
  parameter directly, rather the cache is implemented indirectly in the
  tlsmgr(8) daemon. This means that per-smtp-instance master.cf overrides
  of this parameter are not effective. Note that each of the cache databases
  supported by tlsmgr(8) daemon: $smtpd_tls_session_cache_database,
  $smtp_tls_session_cache_database (and with Postfix 2.3 and later
  $lmtp_tls_session_cache_database), needs to be stored separately. It is not at
  this time possible to store multiple caches in a single database.
Note: dbm databases are not suitable. TLS session objects
    are too large.
As of version 2.5, Postfix no longer uses root privileges when
    opening this file. The file should now be stored under the Postfix-owned
    data_directory. As a migration aid, an attempt to open the file under a
    non-Postfix directory is redirected to the Postfix-owned data_directory, and
    a warning is logged.
Example:
smtp_tls_session_cache_database = btree:/var/db/postfix/smtp_scache
This feature is available in Postfix 2.2 and later.
The expiration time of Postfix SMTP client TLS session cache information. A
  cache cleanup is performed periodically every $smtp_tls_session_cache_timeout
  seconds. As with $smtp_tls_session_cache_database, this parameter is
  implemented in the tlsmgr(8) daemon and therefore per-smtp-instance
  master.cf overrides are not possible.
As of Postfix 2.11 this setting cannot exceed 100 days. If set
    <= 0, session caching is disabled. If set to a positive value less than 2
    minutes, the minimum value of 2 minutes is used instead.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.2 and later.
Zero or more PEM-format files with trust-anchor certificates and/or public keys.
  If the parameter is not empty the root CAs in CAfile and CApath are no longer
  trusted. Rather, the Postfix SMTP client will only trust certificate-chains
  signed by one of the trust-anchors contained in the chosen files. The
  specified trust-anchor certificates and public keys are not subject to
  expiration, and need not be (self-signed) root CAs. They may, if desired, be
  intermediate certificates. Therefore, these certificates also may be found
  "in the middle" of the trust chain presented by the remote SMTP
  server, and any untrusted issuing parent certificates will be ignored. Specify
  a list of pathnames separated by comma or whitespace.
Whether specified in main.cf, or on a per-destination basis, the
    trust-anchor PEM file must be accessible to the Postfix SMTP client in the
    chroot jail if applicable. The trust-anchor file should contain only
    certificates and public keys, no private key material, and must be readable
    by the non-privileged $mail_owner user. This allows destinations to be bound
    to a set of specific CAs or public keys without trusting the same CAs for
    all destinations.
The main.cf parameter supports single-purpose Postfix
    installations that send mail to a fixed set of SMTP peers. At most sites, if
    trust-anchor files are used at all, they will be specified on a
    per-destination basis via the "tafile" attribute of the
    "verify" and "secure" levels in
  smtp_tls_policy_maps.
The underlying mechanism is in support of RFC 7672 (DANE TLSA),
    which defines mechanisms for an SMTP client MTA to securely determine server
    TLS certificates via DNS.
If you want your trust anchors to be public keys, with OpenSSL you
    can extract a single PEM public key from a PEM X.509 file containing a
    single certificate, as follows:
  
$ openssl x509 -in cert.pem -out ta-key.pem -noout -pubkey
This feature is available in Postfix 2.11 and later.
How the Postfix SMTP client verifies the server certificate peername for the
  "verify" TLS security level. In a "verify" TLS policy
  table ($smtp_tls_policy_maps) entry the optional "match" attribute
  overrides this main.cf setting.
This parameter specifies one or more patterns or strategies
    separated by commas, whitespace or colons. In the policy table the only
    valid separator is the colon character.
Patterns specify domain names, or domain name suffixes:
  - example.com
- Match the example.com domain, i.e. one of the names in the server
      certificate must be example.com. Upper and lower case distinctions
      are ignored.
    
 
- .example.com
- Match subdomains of the example.com domain, i.e. match a name in
      the server certificate that consists of a non-zero number of labels
      followed by a .example.com suffix. Case distinctions are ignored.
    
 
Strategies specify a transformation from the next-hop domain to
    the expected name in the server certificate:
  - nexthop
- Match against the next-hop domain, which is either the recipient domain,
      or the transport next-hop configured for the domain stripped of any
      optional socket type prefix, enclosing square brackets and trailing port.
      When MX lookups are not suppressed, this is the original nexthop domain
      prior to the MX lookup, not the result of the MX lookup. For LMTP delivery
      via UNIX-domain sockets, the verified next-hop name is $myhostname. This
      strategy is suitable for use with the "secure" policy. Case is
      ignored.
    
 
- dot-nexthop
- As above, but match server certificate names that are subdomains of the
      next-hop domain. Case is ignored.
    
 
- hostname
- Match against the hostname of the server, often obtained via an
      unauthenticated DNS MX lookup. For LMTP delivery via UNIX-domain sockets,
      the verified name is $myhostname. This matches the verification strategy
      of the "MUST" keyword in the obsolete smtp_tls_per_site table,
      and is suitable for use with the "verify" security level. When
      the next-hop name is enclosed in square brackets to suppress MX lookups,
      the "hostname" strategy is the same as the "nexthop"
      strategy. Case is ignored.
    
 
Sample main.cf setting:
smtp_tls_verify_cert_match = hostname, nexthop, dot-nexthop
Sample policy table override:
example.com     verify  match=hostname:nexthop
.example.com    verify  match=example.com:.example.com:hostname
This feature is available in Postfix 2.3 and later.
Request that the Postfix SMTP client connects using the SUBMISSIONS/SMTPS
  protocol instead of using the STARTTLS command.
This mode requires "smtp_tls_security_level = encrypt"
    or stronger.
Example: deliver all remote mail via a provider's server
    "mail.example.com".
/etc/postfix/main.cf:
    # Client-side SMTPS requires "encrypt" or stronger.
    smtp_tls_security_level = encrypt
    smtp_tls_wrappermode = yes
    # The [] suppress MX lookups.
    relayhost = [mail.example.com]:465
More examples are in TLS_README, including examples for older
    Postfix versions.
This feature is available in Postfix 3.0 and later.
Opportunistic mode: use TLS when a remote SMTP server announces STARTTLS
  support, otherwise send the mail in the clear. Beware: some SMTP servers offer
  STARTTLS even if it is not configured. With Postfix < 2.3, if the TLS
  handshake fails, and no other server is available, delivery is deferred and
  mail stays in the queue. If this is a concern for you, use the
  smtp_tls_per_site feature instead.
This feature is available in Postfix 2.2 and later. With Postfix
    2.3 and later use smtp_tls_security_level instead.
The Postfix SMTP client time limit for sending the XFORWARD command, and for
  receiving the remote SMTP server response.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
What remote SMTP clients are allowed to specify the XVERP command. This command
  requests that mail be delivered one recipient at a time with a per recipient
  return address.
By default, no clients are allowed to specify XVERP.
This parameter was renamed with Postfix version 2.1. The default
    value is backwards compatible with Postfix version 2.0.
Specify a list of network/netmask patterns, separated by commas
    and/or whitespace. The mask specifies the number of bits in the network part
    of a host address. You can also specify hostnames or .domain names (the
    initial dot causes the domain to match any name below it),
    "/file/name" or "type:table" patterns. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a table entry matches a
    lookup string (the lookup result is ignored). Continue long lines by
    starting the next line with whitespace. Specify "!pattern" to
    exclude an address or network block from the list. The form
    "!/file/name" is supported only in Postfix version 2.4 and
  later.
Note: IP version 6 address information must be specified inside []
    in the smtpd_authorized_verp_clients value, and in files specified with
    "/file/name". IP version 6 addresses contain the ":"
    character, and would otherwise be confused with a "type:table"
    pattern.
What remote SMTP clients are allowed to use the XCLIENT feature. This command
  overrides remote SMTP client information that is used for access control.
  Typical use is for SMTP-based content filters, fetchmail-like programs, or
  SMTP server access rule testing. See the XCLIENT_README document for details.
This feature is available in Postfix 2.1 and later.
By default, no clients are allowed to specify XCLIENT.
Specify a list of network/netmask patterns, separated by commas
    and/or whitespace. The mask specifies the number of bits in the network part
    of a host address. You can also specify hostnames or .domain names (the
    initial dot causes the domain to match any name below it),
    "/file/name" or "type:table" patterns. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a table entry matches a
    lookup string (the lookup result is ignored). Continue long lines by
    starting the next line with whitespace. Specify "!pattern" to
    exclude an address or network block from the list. The form
    "!/file/name" is supported only in Postfix version 2.4 and
  later.
Note: IP version 6 address information must be specified inside []
    in the smtpd_authorized_xclient_hosts value, and in files specified with
    "/file/name". IP version 6 addresses contain the ":"
    character, and would otherwise be confused with a "type:table"
    pattern.
What remote SMTP clients are allowed to use the XFORWARD feature. This command
  forwards information that is used to improve logging after SMTP-based content
  filters. See the XFORWARD_README document for details.
This feature is available in Postfix 2.1 and later.
By default, no clients are allowed to specify XFORWARD.
Specify a list of network/netmask patterns, separated by commas
    and/or whitespace. The mask specifies the number of bits in the network part
    of a host address. You can also specify hostnames or .domain names (the
    initial dot causes the domain to match any name below it),
    "/file/name" or "type:table" patterns. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a table entry matches a
    lookup string (the lookup result is ignored). Continue long lines by
    starting the next line with whitespace. Specify "!pattern" to
    exclude an address or network block from the list. The form
    "!/file/name" is supported only in Postfix version 2.4 and
  later.
Note: IP version 6 address information must be specified inside []
    in the smtpd_authorized_xforward_hosts value, and in files specified with
    "/file/name". IP version 6 addresses contain the ":"
    character, and would otherwise be confused with a "type:table"
    pattern.
The text that follows the 220 status code in the SMTP greeting banner. Some
  people like to see the mail version advertised. By default, Postfix shows no
  version.
You MUST specify $myhostname at the start of the text. This is
    required by the SMTP protocol.
Example:
smtpd_banner = $myhostname ESMTP $mail_name ($mail_version)
The maximal number of AUTH commands that any client is allowed to send to this
  service per time unit, regardless of whether or not Postfix actually accepts
  those commands. The time unit is specified with the anvil_rate_time_unit
  configuration parameter.
By default, there is no limit on the number of AUTH commands that
    a client may send.
To disable this feature, specify a limit of 0.
WARNING: The purpose of this feature is to limit abuse. It must
    not be used to regulate legitimate mail traffic.
This feature is available in Postfix 3.1 and later.
How many simultaneous connections any client is allowed to make to this service.
  By default, the limit is set to half the default process limit value.
To disable this feature, specify a limit of 0.
WARNING: The purpose of this feature is to limit abuse. It must
    not be used to regulate legitimate mail traffic.
This feature is available in Postfix 2.2 and later.
The maximal number of connection attempts any client is allowed to make to this
  service per time unit. The time unit is specified with the
  anvil_rate_time_unit configuration parameter.
By default, a client can make as many connections per time unit as
    Postfix can accept.
To disable this feature, specify a limit of 0.
WARNING: The purpose of this feature is to limit abuse. It must
    not be used to regulate legitimate mail traffic.
This feature is available in Postfix 2.2 and later.
Example:
smtpd_client_connection_rate_limit = 1000
Clients that are excluded from smtpd_client_*_count/rate_limit restrictions. See
  the mynetworks parameter description for the parameter value syntax.
By default, clients in trusted networks are excluded. Specify a
    list of network blocks, hostnames or .domain names (the initial dot causes
    the domain to match any name below it).
Note: IP version 6 address information must be specified inside []
    in the smtpd_client_event_limit_exceptions value, and in files specified
    with "/file/name". IP version 6 addresses contain the
    ":" character, and would otherwise be confused with a
    "type:table" pattern.
Pattern matching of domain names is controlled by the presence or
    absence of "smtpd_client_event_limit_exceptions" in the
    parent_domain_matches_subdomains parameter value (Postfix 3.0 and
  later).
This feature is available in Postfix 2.2 and later.
Aggregate smtpd_client_*_count and smtpd_client_*_rate statistics by IPv4
  network blocks with the specified network prefix. Aggregation uses fewer
  anvil(8) resources to maintain counters. By default, aggregation is
  disabled for IPv4.
This feature is available in Postfix 3.8 and later.
Aggregate smtpd_client_*_count and smtpd_client_*_rate statistics by IPv6
  network blocks with the specified network prefix. Aggregation uses fewer the
  anvil(8) resources to maintain counters. By default, aggregation is
  enabled for IPv6.
This feature is available in Postfix 3.8 and later.
The maximal number of message delivery requests that any client is allowed to
  make to this service per time unit, regardless of whether or not Postfix
  actually accepts those messages. The time unit is specified with the
  anvil_rate_time_unit configuration parameter.
By default, a client can send as many message delivery requests
    per time unit as Postfix can accept.
To disable this feature, specify a limit of 0.
WARNING: The purpose of this feature is to limit abuse. It must
    not be used to regulate legitimate mail traffic.
This feature is available in Postfix 2.2 and later.
Example:
smtpd_client_message_rate_limit = 1000
The maximal number of new (i.e., uncached) TLS sessions that a remote SMTP
  client is allowed to negotiate with this service per time unit. The time unit
  is specified with the anvil_rate_time_unit configuration parameter.
By default, a remote SMTP client can negotiate as many new TLS
    sessions per time unit as Postfix can accept.
To disable this feature, specify a limit of 0. Otherwise, specify
    a limit that is at least the per-client concurrent session limit, or else
    legitimate client sessions may be rejected.
WARNING: The purpose of this feature is to limit abuse. It must
    not be used to regulate legitimate mail traffic.
This feature is available in Postfix 2.3 and later.
Example:
smtpd_client_new_tls_session_rate_limit = 100
Enable logging of the remote SMTP client port in addition to the hostname and IP
  address. The logging format is "host[address]:port".
This feature is available in Postfix 2.5 and later.
The maximal number of recipient addresses that any client is allowed to send to
  this service per time unit, regardless of whether or not Postfix actually
  accepts those recipients. The time unit is specified with the
  anvil_rate_time_unit configuration parameter.
By default, a client can send as many recipient addresses per time
    unit as Postfix can accept.
To disable this feature, specify a limit of 0.
WARNING: The purpose of this feature is to limit abuse. It must
    not be used to regulate legitimate mail traffic.
This feature is available in Postfix 2.2 and later.
Example:
smtpd_client_recipient_rate_limit = 1000
Optional restrictions that the Postfix SMTP server applies in the context of a
  client connection request. See SMTPD_ACCESS_README, section "Delayed
  evaluation of SMTP access restriction lists" for a discussion of
  evaluation context and time.
The default is to allow all connection requests.
Specify a list of restrictions, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace.
    Restrictions are applied in the order as specified; the first restriction
    that matches wins.
The following restrictions are specific to client hostname or
    client network address information.
  - check_ccert_access type:table
- By default use the remote SMTP client certificate fingerprint or the
      public key fingerprint (Postfix 2.9 and later) as the lookup key for the
      specified access(5) database; with Postfix version 2.2, also
      require that the remote SMTP client certificate is verified successfully.
      The fingerprint digest algorithm is configurable via the
      smtpd_tls_fingerprint_digest parameter (hard-coded as md5 prior to Postfix
      version 2.5). This feature requires "smtpd_tls_ask_ccert = yes"
      and is available with Postfix version 2.2 and later.
    
 The default algorithm is sha256 with Postfix >= 3.6 and the
      compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
      the default algorithm is md5. The best-practice algorithm is now
      sha256. Recent advances in hash function cryptanalysis have led to
      md5 and sha1 being deprecated in favor of sha256. However, as long as
      there are no known "second pre-image" attacks against the older
      algorithms, their use in this context, though not recommended, is still
      likely safe.
 Alternatively, check_ccert_access accepts an explicit search order (Postfix
      3.5 and later). The default search order as described above corresponds
      with:
 check_ccert_access { type:table, { search_order = cert_fingerprint,
      pubkey_fingerprint } }
 The commas are optional.
 
- check_client_access type:table
- Search the specified access database for the client hostname, parent
      domains, client IP address, or networks obtained by stripping least
      significant octets. See the access(5) manual page for details.
    
 
- check_client_a_access type:table
- Search the specified access(5) database for the IP addresses for
      the client hostname, and execute the corresponding action. Note: a result
      of "OK" is not allowed for safety reasons. Instead, use DUNNO in
      order to exclude specific hosts from denylists. This feature is available
      in Postfix 3.0 and later.
    
 
- check_client_mx_access type:table
- Search the specified access(5) database for the MX hosts for the
      client hostname, and execute the corresponding action. If no MX record is
      found, look up A or AAAA records, just like the Postfix SMTP client would.
      Note: a result of "OK" is not allowed for safety reasons.
      Instead, use DUNNO in order to exclude specific hosts from denylists. This
      feature is available in Postfix 2.7 and later.
    
 
- check_client_ns_access type:table
- Search the specified access(5) database for the DNS servers for the
      client hostname, and execute the corresponding action. Note: a result of
      "OK" is not allowed for safety reasons. Instead, use DUNNO in
      order to exclude specific hosts from denylists. This feature is available
      in Postfix 2.7 and later.
    
 
- check_reverse_client_hostname_access type:table
- Search the specified access database for the unverified reverse client
      hostname, parent domains, client IP address, or networks obtained by
      stripping least significant octets. See the access(5) manual page
      for details. Note: a result of "OK" is not allowed for safety
      reasons. Instead, use DUNNO in order to exclude specific hosts from
      denylists. This feature is available in Postfix 2.6 and later.
    
 
- check_reverse_client_hostname_a_access type:table
- Search the specified access(5) database for the IP addresses for
      the unverified reverse client hostname, and execute the corresponding
      action. Note: a result of "OK" is not allowed for safety
      reasons. Instead, use DUNNO in order to exclude specific hosts from
      denylists. This feature is available in Postfix 3.0 and later.
    
 
- check_reverse_client_hostname_mx_access type:table
- Search the specified access(5) database for the MX hosts for the
      unverified reverse client hostname, and execute the corresponding action.
      If no MX record is found, look up A or AAAA records, just like the Postfix
      SMTP client would. Note: a result of "OK" is not allowed for
      safety reasons. Instead, use DUNNO in order to exclude specific hosts from
      denylists. This feature is available in Postfix 2.7 and later.
    
 
- check_reverse_client_hostname_ns_access type:table
- Search the specified access(5) database for the DNS servers for the
      unverified reverse client hostname, and execute the corresponding action.
      Note: a result of "OK" is not allowed for safety reasons.
      Instead, use DUNNO in order to exclude specific hosts from denylists. This
      feature is available in Postfix 2.7 and later.
    
 
- check_sasl_access type:table
- Use the remote SMTP client SASL user name as the lookup key for the
      specified access(5) database. The lookup key has the form
      "username@domainname" when the smtpd_sasl_local_domain parameter
      value is non-empty. Unlike the check_client_access feature,
      check_sasl_access does not perform matches of parent domains or IP subnet
      ranges. This feature is available with Postfix version 2.11 and later.
    
 
- permit_inet_interfaces
- Permit the request when the client IP address matches $inet_interfaces.
    
 
- permit_mynetworks
- Permit the request when the client IP address matches any network or
      network address listed in $mynetworks.
    
 
- permit_sasl_authenticated
- Permit the request when the client is successfully authenticated via the
      RFC 4954 (AUTH) protocol.
    
 
- permit_tls_all_clientcerts
- Permit the request when the remote SMTP client certificate is verified
      successfully. This option must be used only if a special CA issues the
      certificates and only this CA is listed as a trusted CA. Otherwise,
      clients with a third-party certificate would also be allowed to relay.
      Specify "tls_append_default_CA = no" when the trusted CA is
      specified with smtpd_tls_CAfile or smtpd_tls_CApath, to prevent Postfix
      from appending the system-supplied default CAs. This feature requires
      "smtpd_tls_ask_ccert = yes" and is available with Postfix
      version 2.2 and later.
    
 
- permit_tls_clientcerts
- Permit the request when the remote SMTP client certificate fingerprint or
      public key fingerprint (Postfix 2.9 and later) is listed in
      $relay_clientcerts. The fingerprint digest algorithm is configurable via
      the smtpd_tls_fingerprint_digest parameter (hard-coded as md5 prior to
      Postfix version 2.5). This feature requires "smtpd_tls_ask_ccert =
      yes" and is available with Postfix version 2.2 and later.
    
 The default algorithm is sha256 with Postfix >= 3.6 and the
      compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
      the default algorithm is md5. The best-practice algorithm is now
      sha256. Recent advances in hash function cryptanalysis have led to
      md5 and sha1 being deprecated in favor of sha256. However, as long as
      there are no known "second pre-image" attacks against the older
      algorithms, their use in this context, though not recommended, is still
      likely safe.
 
- reject_rbl_client rbl_domain=d.d.d.d
- Reject the request when the reversed client network address is listed with
      the A record "d.d.d.d" under rbl_domain (Postfix
      version 2.1 and later only). Each "d" is a number, or a
      pattern inside "[]" that contains one or more
      ";"-separated numbers or number..number ranges (Postfix version
      2.8 and later). If no "=d.d.d.d" is specified, reject the
      request when the reversed client network address is listed with any A
      record under rbl_domain.
    
 The maps_rbl_reject_code parameter specifies the response code for rejected
      requests (default: 554), the default_rbl_reply parameter specifies the
      default server reply, and the rbl_reply_maps parameter specifies tables
      with server replies indexed by rbl_domain. This feature is
      available in Postfix 2.0 and later.
 
- permit_dnswl_client dnswl_domain=d.d.d.d
- Accept the request when the reversed client network address is listed with
      the A record "d.d.d.d" under dnswl_domain. Each
      "d" is a number, or a pattern inside "[]" that
      contains one or more ";"-separated numbers or number..number
      ranges. If no "=d.d.d.d" is specified, accept the request
      when the reversed client network address is listed with any A record under
      dnswl_domain.
    
 For safety, permit_dnswl_client is silently ignored when it would override
      reject_unauth_destination. The result is DEFER_IF_REJECT when allowlist
      lookup fails. This feature is available in Postfix 2.8 and later.
 
- reject_rhsbl_client rbl_domain=d.d.d.d
- Reject the request when the client hostname is listed with the A record
      "d.d.d.d" under rbl_domain (Postfix version 2.1
      and later only). Each "d" is a number, or a pattern
      inside "[]" that contains one or more ";"-separated
      numbers or number..number ranges (Postfix version 2.8 and later). If no
      "=d.d.d.d" is specified, reject the request when the
      client hostname is listed with any A record under rbl_domain. See
      the reject_rbl_client description above for additional RBL related
      configuration parameters. This feature is available in Postfix 2.0 and
      later; with Postfix version 2.8 and later, reject_rhsbl_reverse_client
      will usually produce better results.
    
 
- permit_rhswl_client rhswl_domain=d.d.d.d
- Accept the request when the client hostname is listed with the A record
      "d.d.d.d" under rhswl_domain. Each
      "d" is a number, or a pattern inside "[]" that
      contains one or more ";"-separated numbers or number..number
      ranges. If no "=d.d.d.d" is specified, accept the request
      when the client hostname is listed with any A record under
      rhswl_domain.
    
 Caution: client name allowlisting is fragile, since the client name lookup
      can fail due to temporary outages. Client name allowlisting should be used
      only to reduce false positives in e.g. DNS-based blocklists, and not for
      making access rule exceptions.
 For safety, permit_rhswl_client is silently ignored when it would override
      reject_unauth_destination. The result is DEFER_IF_REJECT when allowlist
      lookup fails. This feature is available in Postfix 2.8 and later.
 
- reject_rhsbl_reverse_client rbl_domain=d.d.d.d
- Reject the request when the unverified reverse client hostname is listed
      with the A record "d.d.d.d" under rbl_domain. Each
      "d" is a number, or a pattern inside "[]" that
      contains one or more ";"-separated numbers or number..number
      ranges. If no "=d.d.d.d" is specified, reject the request
      when the unverified reverse client hostname is listed with any A record
      under rbl_domain. See the reject_rbl_client description above for
      additional RBL related configuration parameters. This feature is available
      in Postfix 2.8 and later.
    
 
- reject_unknown_client_hostname (with Postfix < 2.3:
    reject_unknown_client)
- Reject the request when 1) the client IP address->name mapping fails,
      or 2) the name->address mapping fails, or 3) the name->address
      mapping does not match the client IP address.
    
 This is a stronger restriction than the
      reject_unknown_reverse_client_hostname feature, which triggers only under
      condition 1) above.
 The unknown_client_reject_code parameter specifies the response code for
      rejected requests (default: 450). The reply is always 450 in case the
      address->name or name->address lookup failed due to a temporary
      problem.
 
- reject_unknown_reverse_client_hostname
- Reject the request when the client IP address has no address->name
      mapping.
    
 This is a weaker restriction than the reject_unknown_client_hostname
      feature, which requires not only that the address->name and
      name->address mappings exist, but also that the two mappings reproduce
      the client IP address.
 The unknown_client_reject_code parameter specifies the response code for
      rejected requests (default: 450). The reply is always 450 in case the
      address->name lookup failed due to a temporary problem.
 This feature is available in Postfix 2.3 and later.
 
In addition, you can use any of the following generic
    restrictions. These restrictions are applicable in any SMTP command
  context.
  - check_policy_service servername
- Query the specified policy server. See the SMTPD_POLICY_README document
      for details. This feature is available in Postfix 2.1 and later.
    
 
- defer
- Defer the request. The client is told to try again later. This restriction
      is useful at the end of a restriction list, to make the default policy
      explicit.
    
 The defer_code parameter specifies the SMTP server reply code (default:
      450).
 
- defer_if_permit
- Defer the request if some later restriction would result in an explicit or
      implicit PERMIT action. This is useful when a denylisting feature fails
      due to a temporary problem. This feature is available in Postfix version
      2.1 and later.
    
 
- defer_if_reject
- Defer the request if some later restriction would result in a REJECT
      action. This is useful when an allowlisting feature fails due to a
      temporary problem. This feature is available in Postfix version 2.1 and
      later.
    
 
- permit
- Permit the request. This restriction is useful at the end of a restriction
      list, to make the default policy explicit.
    
 
- reject_multi_recipient_bounce
- Reject the request when the envelope sender is the null address, and the
      message has multiple envelope recipients. This usage has rare but
      legitimate applications: under certain conditions, multi-recipient mail
      that was posted with the DSN option NOTIFY=NEVER may be forwarded with the
      null sender address.
    
 Note: this restriction can only work reliably when used in
      smtpd_data_restrictions or smtpd_end_of_data_restrictions, because the
      total number of recipients is not known at an earlier stage of the SMTP
      conversation. Use at the RCPT stage will only reject the second etc.
      recipient.
 The multi_recipient_bounce_reject_code parameter specifies the response code
      for rejected requests (default: 550). This feature is available in Postfix
      2.1 and later.
 
- reject_plaintext_session
- Reject the request when the connection is not encrypted. This restriction
      should not be used before the client has had a chance to negotiate
      encryption with the AUTH or STARTTLS commands.
    
 The plaintext_reject_code parameter specifies the response code for rejected
      requests (default: 450). This feature is available in Postfix 2.3 and
      later.
 
- reject_unauth_pipelining
- Reject the request when the client sends SMTP commands ahead of time where
      it is not allowed, or when the client sends SMTP commands ahead of time
      without knowing that Postfix actually supports ESMTP command pipelining.
      This stops mail from bulk mail software that improperly uses ESMTP command
      pipelining in order to speed up deliveries.
    
 With Postfix 2.6 and later, the SMTP server sets a per-session flag whenever
      it detects illegal pipelining, including pipelined HELO or EHLO commands.
      The reject_unauth_pipelining feature simply tests whether the flag was set
      at any point in time during the session.
 With older Postfix versions, reject_unauth_pipelining checks the current
      status of the input read queue, and its usage is not recommended in
      contexts other than smtpd_data_restrictions.
 
- reject
- Reject the request. This restriction is useful at the end of a restriction
      list, to make the default policy explicit. The reject_code configuration
      parameter specifies the response code for rejected requests (default:
      554).
    
 
- sleep seconds
- Pause for the specified number of seconds and proceed with the next
      restriction in the list, if any. This may stop zombie mail when used as:
    
/etc/postfix/main.cf:
    smtpd_client_restrictions =
        sleep 1, reject_unauth_pipelining
    smtpd_delay_reject = no
    This feature is available in Postfix 2.3.
 
- warn_if_reject
- A safety net for testing. When "warn_if_reject" is placed before
      a reject-type restriction, access table query, or check_policy_service
      query, this logs a "reject_warning" message instead of rejecting
      a request (when a reject-type restriction fails due to a temporary error,
      this logs a "reject_warning" message for any implicit
      "defer_if_permit" actions that would normally prevent mail from
      being accepted by some later access restriction). This feature has no
      effect on defer_if_reject restrictions.
    
 
Other restrictions that are valid in this context:
  - •
- SMTP command specific restrictions that are described under the
      smtpd_helo_restrictions, smtpd_sender_restrictions or
      smtpd_recipient_restrictions parameters. When helo, sender or recipient
      restrictions are listed under smtpd_client_restrictions, they have effect
      only with "smtpd_delay_reject = yes", so that
      $smtpd_client_restrictions is evaluated at the time of the RCPT TO
      command.
    
 
Example:
smtpd_client_restrictions = permit_mynetworks, reject_unknown_client_hostname
A mechanism to transform commands from remote SMTP clients. This is a
  last-resort tool to work around client commands that break interoperability
  with the Postfix SMTP server. Other uses involve fault injection to test
  Postfix's handling of invalid commands.
Specify the name of a "type:table" lookup table. The
    search string is the SMTP command as received from the remote SMTP client,
    except that initial whitespace and the trailing <CR><LF> are
    removed. The result value is executed by the Postfix SMTP server.
There is no need to use smtpd_command_filter for the following
    cases:
  - Use "resolve_numeric_domain = yes" to accept
      "user@ipaddress".
- Postfix already accepts the correct form
      "user@[ipaddress]". Use virtual_alias_maps or
      canonical_maps to translate these into domain names if necessary.
- Use "strict_rfc821_envelopes = no" to accept "RCPT
      TO:<User Name <user@example.com>>". Postfix
      will ignore the "User Name" part and deliver to
      the <user@example.com> address.
    
 
Examples of problems that can be solved with the
    smtpd_command_filter feature:
/etc/postfix/main.cf:
    smtpd_command_filter = pcre:/etc/postfix/command_filter
/etc/postfix/command_filter:
    # Work around clients that send malformed HELO commands.
    /^HELO\s*$/ HELO domain.invalid
    # Work around clients that send empty lines.
    /^\s*$/     NOOP
    # Work around clients that send RCPT TO:<'user@domain'>.
    # WARNING: do not lose the parameters that follow the address.
    /^(RCPT\s+TO:\s*<)'([^[:space:]]+)'(>.*)/     $1$2$3
    # Append XVERP to MAIL FROM commands to request VERP-style delivery.
    # See VERP_README for more information on how to use Postfix VERP.
    /^(MAIL\s+FROM:\s*<listname@example\.com>.*)/   $1 XVERP
    # Bounce-never mail sink. Use notify_classes=bounce,resource,software
    # to send bounced mail to the postmaster (with message body removed).
    /^(RCPT\s+TO:\s*<.*>.*)\s+NOTIFY=\S+(.*)/     $1 NOTIFY=NEVER$2
    /^(RCPT\s+TO:.*)/                             $1 NOTIFY=NEVER
This feature is available in Postfix 2.7.
Optional access restrictions that the Postfix SMTP server applies in the context
  of the SMTP DATA command. See SMTPD_ACCESS_README, section "Delayed
  evaluation of SMTP access restriction lists" for a discussion of
  evaluation context and time.
This feature is available in Postfix 2.0 and later.
Specify a list of restrictions, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace.
    Restrictions are applied in the order as specified; the first restriction
    that matches wins.
The following restrictions are valid in this context:
  - Generic restrictions that can be used in any SMTP command context,
      described under smtpd_client_restrictions.
- SMTP command specific restrictions described under
      smtpd_client_restrictions, smtpd_helo_restrictions,
      smtpd_sender_restrictions or smtpd_recipient_restrictions.
- However, no recipient information is available in the case of
      multi-recipient mail. Acting on only one recipient would be misleading,
      because any decision will affect all recipients equally. Acting on all
      recipients would require a possibly very large amount of memory, and would
      also be misleading for the reasons mentioned before.
    
 
Examples:
smtpd_data_restrictions = reject_unauth_pipelining
smtpd_data_restrictions = reject_multi_recipient_bounce
Postpone the start of an SMTP mail transaction until a valid RCPT TO command is
  received. Specify "no" to create a mail transaction as soon as the
  Postfix SMTP server receives a valid MAIL FROM command.
With sites that reject lots of mail, the default setting reduces
    the use of disk, CPU and memory resources. The downside is that rejected
    recipients are logged with NOQUEUE instead of a mail transaction ID. This
    complicates the logfile analysis of multi-recipient mail.
This feature is available in Postfix 2.3 and later.
Wait until the RCPT TO command before evaluating $smtpd_client_restrictions,
  $smtpd_helo_restrictions and $smtpd_sender_restrictions, or wait until the
  ETRN command before evaluating $smtpd_client_restrictions and
  $smtpd_helo_restrictions.
This feature is turned on by default because some clients
    apparently mis-behave when the Postfix SMTP server rejects commands before
    RCPT TO.
The default setting has one major benefit: it allows Postfix to
    log recipient address information when rejecting a client name/address or
    sender address, so that it is possible to find out whose mail is being
    rejected.
Lookup tables, indexed by the remote SMTP client address, with case insensitive
  lists of EHLO keywords (pipelining, starttls, auth, etc.) that the Postfix
  SMTP server will not send in the EHLO response to a remote SMTP client. See
  smtpd_discard_ehlo_keywords for details. The tables are not searched by
  hostname for robustness reasons.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
This feature is available in Postfix 2.2 and later.
A case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that
  the Postfix SMTP server will not send in the EHLO response to a remote SMTP
  client.
This feature is available in Postfix 2.2 and later.
Notes:
  - Specify the silent-discard pseudo keyword to prevent this action
      from being logged.
- Use the smtpd_discard_ehlo_keyword_address_maps feature to discard EHLO
      keywords selectively.
    
 
Optional filter for Postfix SMTP server DNS lookup results. See
  smtp_dns_reply_filter for details including an example.This feature is available in Postfix 3.0 and later.
Optional access restrictions that the Postfix SMTP server applies in the context
  of the SMTP END-OF-DATA command. See SMTPD_ACCESS_README, section
  "Delayed evaluation of SMTP access restriction lists" for a
  discussion of evaluation context and time.
This feature is available in Postfix 2.2 and later.
See smtpd_data_restrictions for details and limitations.
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and require
  that clients use TLS encryption. According to RFC 2487 this MUST NOT be
  applied in case of a publicly-referenced SMTP server. This option is therefore
  off by default.
Note 1: "smtpd_enforce_tls = yes" implies
    "smtpd_tls_auth_only = yes".
Note 2: when invoked via "sendmail -bs", Postfix
    will never offer STARTTLS due to insufficient privileges to access the
    server private key. This is intended behavior.
This feature is available in Postfix 2.2 and later. With Postfix
    2.3 and later use smtpd_tls_security_level instead.
With Postfix version 2.1 and later: the SMTP server response delay after a
  client has made more than $smtpd_soft_error_limit errors, and fewer than
  $smtpd_hard_error_limit errors, without delivering mail.
With Postfix version 2.0 and earlier: the SMTP server delay before
    sending a reject (4xx or 5xx) response, when the client has made fewer than
    $smtpd_soft_error_limit errors without delivering mail. When the client has
    made $smtpd_soft_error_limit or more errors, delay all responses with the
    larger of (number of errors) seconds or $smtpd_error_sleep_time.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
Optional restrictions that the Postfix SMTP server applies in the context of a
  client ETRN command. See SMTPD_ACCESS_README, section "Delayed evaluation
  of SMTP access restriction lists" for a discussion of evaluation context
  and time.
The Postfix ETRN implementation accepts only destinations that are
    eligible for the Postfix "fast flush" service. See the ETRN_README
    file for details.
Specify a list of restrictions, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace.
    Restrictions are applied in the order as specified; the first restriction
    that matches wins.
The following restrictions are specific to the domain name
    information received with the ETRN command.
  - check_etrn_access type:table
- Search the specified access database for the ETRN domain name or its
      parent domains. See the access(5) manual page for details.
    
 
Other restrictions that are valid in this context:
  - Generic restrictions that can be used in any SMTP command context,
      described under smtpd_client_restrictions.
- SMTP command specific restrictions described under
      smtpd_client_restrictions and smtpd_helo_restrictions.
    
 
Example:
smtpd_etrn_restrictions = permit_mynetworks, reject
What characters are allowed in $name expansions of RBL reply templates.
  Characters not in the allowed set are replaced by "_". Use C like
  escapes to specify special characters such as whitespace.
The smtpd_expansion_filter value is not subject to Postfix
    configuration parameter $name expansion.
This feature is available in Postfix 2.0 and later.
Reply with "Error: bare <LF> received" and disconnect when a
  remote SMTP client sends a line ending in <LF>, violating the RFC 5321
  requirement that lines must end in <CR><LF>. This feature is
  disbled by default with Postfix < 3.9. Use
  smtpd_forbid_bare_newline_exclusions to exclude non-standard clients such as
  netcat. Specify "smtpd_forbid_bare_newline = no" to disable (not
  recommended for an Internet-connected MTA).
See https://www.postfix.org/smtp-smuggling.html for details.
Example:
  
# Disconnect remote SMTP clients that send bare newlines, but allow
# local clients with non-standard SMTP implementations such as netcat,
# fax machines, or load balancer health checks.
#
smtpd_forbid_bare_newline = yes
smtpd_forbid_bare_newline_exclusions = $mynetworks
This feature is available in Postfix >= 3.9, 3.8.4, 3.7.9,
    3.6.13, and 3.5.23.
Exclude the specified clients from smtpd_forbid_bare_newline enforcement. It
  uses the same syntax and parent-domain matching behavior as mynetworks.
Example:
  
# Disconnect remote SMTP clients that send bare newlines, but allow
# local clients with non-standard SMTP implementations such as netcat,
# fax machines, or load balancer health checks.
#
smtpd_forbid_bare_newline = yes
smtpd_forbid_bare_newline_exclusions = $mynetworks
This feature is available in Postfix >= 3.9, 3.8.4, 3.7.9,
    3.6.13, and 3.5.23.
Disconnect remote SMTP clients that violate RFC 2920 (or 5321) command
  pipelining constraints. The server replies with "554 5.5.0 Error: SMTP
  protocol synchronization" and logs the unexpected remote SMTP client
  input. Specify "smtpd_forbid_unauth_pipelining = yes" to enable.
  This feature is enabled by default with Postfix >= 3.9.
This feature is available in Postfix >= 3.9, 3.8.1, 3.7.6,
    3.6.10, and 3.5.20.
List of commands that cause the Postfix SMTP server to immediately terminate the
  session with a 221 code. This can be used to disconnect clients that obviously
  attempt to abuse the system. In addition to the commands listed in this
  parameter, commands that follow the "Label:" format of message
  headers will also cause a disconnect. With Postfix versions 3.6 and earlier,
  the default value is "CONNECT GET POST".
This feature is available in Postfix 2.2 and later.
Support for inline regular expressions was added in Postfix
    version 3.7. See regexp_table(5) for a description of the syntax and
    features.
The maximal number of errors a remote SMTP client is allowed to make without
  delivering mail. The Postfix SMTP server disconnects when the limit is
  reached. Normally the default limit is 20, but it changes under overload to
  just 1. With Postfix 2.5 and earlier, the SMTP server always allows up to 20
  errors by default. Valid values are greater than zero.
Require that a remote SMTP client introduces itself with the HELO or EHLO
  command before sending the MAIL command or other commands that require EHLO
  negotiation.
Example:
smtpd_helo_required = yes
Optional restrictions that the Postfix SMTP server applies in the context of a
  client HELO command. See SMTPD_ACCESS_README, section "Delayed evaluation
  of SMTP access restriction lists" for a discussion of evaluation context
  and time.
The default is to permit everything.
Note: specify "smtpd_helo_required = yes" to fully
    enforce this restriction (without "smtpd_helo_required = yes", a
    client can simply skip smtpd_helo_restrictions by not sending HELO or
  EHLO).
Specify a list of restrictions, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace.
    Restrictions are applied in the order as specified; the first restriction
    that matches wins.
The following restrictions are specific to the hostname
    information received with the HELO or EHLO command.
  - check_helo_access type:table
- Search the specified access(5) database for the HELO or EHLO
      hostname or parent domains, and execute the corresponding action. Note:
      specify "smtpd_helo_required = yes" to fully enforce this
      restriction (without "smtpd_helo_required = yes", a client can
      simply skip check_helo_access by not sending HELO or EHLO).
    
 
- check_helo_a_access type:table
- Search the specified access(5) database for the IP addresses for
      the HELO or EHLO hostname, and execute the corresponding action. Note 1: a
      result of "OK" is not allowed for safety reasons. Instead, use
      DUNNO in order to exclude specific hosts from denylists. Note 2: specify
      "smtpd_helo_required = yes" to fully enforce this restriction
      (without "smtpd_helo_required = yes", a client can simply skip
      check_helo_a_access by not sending HELO or EHLO). This feature is
      available in Postfix 3.0 and later.
    
 
- check_helo_mx_access type:table
- Search the specified access(5) database for the MX hosts for the
      HELO or EHLO hostname, and execute the corresponding action. If no MX
      record is found, look up A or AAAA records, just like the Postfix SMTP
      client would. Note 1: a result of "OK" is not allowed for safety
      reasons. Instead, use DUNNO in order to exclude specific hosts from
      denylists. Note 2: specify "smtpd_helo_required = yes" to fully
      enforce this restriction (without "smtpd_helo_required = yes", a
      client can simply skip check_helo_mx_access by not sending HELO or EHLO).
      This feature is available in Postfix 2.1 and later.
    
 
- check_helo_ns_access type:table
- Search the specified access(5) database for the DNS servers for the
      HELO or EHLO hostname, and execute the corresponding action. Note 1: a
      result of "OK" is not allowed for safety reasons. Instead, use
      DUNNO in order to exclude specific hosts from denylists. Note 2: specify
      "smtpd_helo_required = yes" to fully enforce this restriction
      (without "smtpd_helo_required = yes", a client can simply skip
      check_helo_ns_access by not sending HELO or EHLO). This feature is
      available in Postfix 2.1 and later.
    
 
- reject_invalid_helo_hostname (with Postfix < 2.3:
    reject_invalid_hostname)
- Reject the request when the HELO or EHLO hostname is malformed. Note:
      specify "smtpd_helo_required = yes" to fully enforce this
      restriction (without "smtpd_helo_required = yes", a client can
      simply skip reject_invalid_helo_hostname by not sending HELO or EHLO).
    
 The invalid_hostname_reject_code specifies the response code for rejected
      requests (default: 501).
 
- reject_non_fqdn_helo_hostname (with Postfix < 2.3:
    reject_non_fqdn_hostname)
- Reject the request when the HELO or EHLO hostname is not in
      fully-qualified domain or address literal form, as required by the RFC.
      Note: specify "smtpd_helo_required = yes" to fully enforce this
      restriction (without "smtpd_helo_required = yes", a client can
      simply skip reject_non_fqdn_helo_hostname by not sending HELO or EHLO).
    
 The non_fqdn_reject_code parameter specifies the response code for rejected
      requests (default: 504).
 
- reject_rhsbl_helo rbl_domain=d.d.d.d
- Reject the request when the HELO or EHLO hostname is listed with the A
      record "d.d.d.d" under rbl_domain (Postfix version
      2.1 and later only). Each "d" is a number, or a pattern
      inside "[]" that contains one or more ";"-separated
      numbers or number..number ranges (Postfix version 2.8 and later). If no
      "=d.d.d.d" is specified, reject the request when the HELO
      or EHLO hostname is listed with any A record under rbl_domain. See
      the reject_rbl_client description for additional RBL related configuration
      parameters. Note: specify "smtpd_helo_required = yes" to fully
      enforce this restriction (without "smtpd_helo_required = yes", a
      client can simply skip reject_rhsbl_helo by not sending HELO or EHLO).
      This feature is available in Postfix 2.0 and later.
    
 
- reject_unknown_helo_hostname (with Postfix < 2.3:
    reject_unknown_hostname)
- Reject the request when the HELO or EHLO hostname has no DNS A or MX
      record.
    
 The reply is specified with the unknown_hostname_reject_code parameter
      (default: 450) or unknown_helo_hostname_tempfail_action (default:
      defer_if_permit). See the respective parameter descriptions for details.
 Note: specify "smtpd_helo_required = yes" to fully enforce this
      restriction (without "smtpd_helo_required = yes", a client can
      simply skip reject_unknown_helo_hostname by not sending HELO or EHLO).
 
Other restrictions that are valid in this context:
  - Generic restrictions that can be used in any SMTP command context,
      described under smtpd_client_restrictions.
- Client hostname or network address specific restrictions described under
      smtpd_client_restrictions.
- SMTP command specific restrictions described under
      smtpd_sender_restrictions or smtpd_recipient_restrictions. When sender or
      recipient restrictions are listed under smtpd_helo_restrictions, they have
      effect only with "smtpd_delay_reject = yes", so that
      $smtpd_helo_restrictions is evaluated at the time of the RCPT TO command.
    
 
Examples:
smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname
smtpd_helo_restrictions = permit_mynetworks, reject_unknown_helo_hostname
The maximal number of lines in the Postfix SMTP server command history before it
  is flushed upon receipt of EHLO, RSET, or end of DATA.
The number of junk commands (NOOP, VRFY, ETRN or RSET) that a remote SMTP client
  can send before the Postfix SMTP server starts to increment the error counter
  with each junk command. The junk command count is reset after mail is
  delivered. See also the smtpd_error_sleep_time and smtpd_soft_error_limit
  configuration parameters. Normally the default limit is 100, but it changes
  under overload to just 1. With Postfix 2.5 and earlier, the SMTP server always
  allows up to 100 junk commands by default.
Enable logging of the named "permit" actions in SMTP server access
  lists (by default, the SMTP server logs "reject" actions but not
  "permit" actions). This feature does not affect conditional actions
  such as "defer_if_permit".
Specify a list of "permit" action names,
    "/file/name" or "type:table" patterns, separated by
    commas and/or whitespace. The list is matched left to right, and the search
    stops on the first match. A "/file/name" pattern is replaced by
    its contents; a "type:table" lookup table is matched when a name
    matches a lookup key (the lookup result is ignored). Continue long lines by
    starting the next line with whitespace. Specify "!pattern" to
    exclude a name from the list.
Examples:
/etc/postfix/main.cf:
    # Log all "permit" actions.
    smtpd_log_access_permit_actions = static:all
/etc/postfix/main.cf:
    # Log "permit_dnswl_client" only.
    smtpd_log_access_permit_actions = permit_dnswl_client
This feature is available in Postfix 2.10 and later.
Lookup tables with Milter settings per remote SMTP client IP address. The lookup
  result overrides the smtpd_milters setting, and has the same syntax.
Note: lookup tables cannot return empty responses. Specify a
    lookup result of DISABLE (case does not matter) to indicate that Milter
    support should be disabled.
Example to disable Milters for local clients:
/etc/postfix/main.cf:
    smtpd_milter_maps = cidr:/etc/postfix/smtpd_milter_map
    smtpd_milters = inet:host:port, { inet:host:port, ... }, ...
/etc/postfix/smtpd_milter_map:
    # Disable Milters for local clients.
    127.0.0.0/8    DISABLE
    192.168.0.0/16 DISABLE
    ::/64          DISABLE
    2001:db8::/32  DISABLE
This feature is available in Postfix 3.2 and later.
A list of Milter (mail filter) applications for new mail that arrives via the
  Postfix smtpd(8) server. Specify space or comma as separator. See the
  MILTER_README document for details.
This feature is available in Postfix 2.3 and later.
The minimum plaintext data transfer rate in bytes/second for DATA and BDAT
  requests, when deadlines are enabled with smtpd_per_request_deadline. After a
  read operation transfers N plaintext message bytes (possibly after TLS
  decryption), and after the DATA or BDAT request deadline is decremented by the
  elapsed time of that read operation, the DATA or BDAT request deadline is
  incremented by N/smtpd_min_data_rate seconds. However, the deadline will never
  be incremented beyond the time limit specified with smtpd_timeout.
This feature is available in Postfix 3.7 and later.
List of commands that the Postfix SMTP server replies to with "250
  Ok", without doing any syntax checks and without changing state. This
  list overrides any commands built into the Postfix SMTP server.
The lookup key to be used in SMTP access(5) tables instead of the null
  sender address.
Attempt to look up the remote SMTP client hostname, and verify that the name
  matches the client IP address. A client name is set to "unknown"
  when it cannot be looked up or verified, or when name lookup is disabled.
  Turning off name lookup reduces delays due to DNS lookup and increases the
  maximal inbound delivery rate.
This feature is available in Postfix 2.3 and later.
Change the behavior of the smtpd_timeout and smtpd_starttls_timeout time limits,
  from a time limit per read or write system call, to a time limit to send or
  receive a complete record (an SMTP command line, SMTP response line, SMTP
  message content line, or TLS protocol message). This limits the impact from
  hostile peers that trickle data one byte at a time.
Note: when per-record deadlines are enabled, a short timeout may
    cause problems with TLS over very slow network connections. The reasons are
    that a TLS protocol message can be up to 16 kbytes long (with TLSv1), and
    that an entire TLS protocol message must be sent or received within the
    per-record deadline.
This feature is available in Postfix 2.9-3.6. With older Postfix
    releases, the behavior is as if this parameter is set to "no".
    Postfix 3.7 and later use smtpd_per_request_deadline.
Change the behavior of the smtpd_timeout and smtpd_starttls_timeout time limits,
  from a time limit per plaintext or TLS read or write call, to a combined time
  limit for receiving a complete SMTP request and for sending a complete SMTP
  response. The deadline limits only the time spent waiting for plaintext or TLS
  read or write calls, not time spent elsewhere. The per-request deadline limits
  the impact from hostile peers that trickle data one byte at a time.
See smtpd_min_data_rate for how the per-request deadline is
    managed during the DATA and BDAT phase.
Note: when per-request deadlines are enabled, a short time limit
    may cause problems with TLS over very slow network connections. The reason
    is that a TLS protocol message can be up to 16 kbytes long (with TLSv1), and
    that an entire TLS protocol message must be transferred within the
    per-request deadline.
This feature is available in Postfix 3.7 and later. A weaker
    feature, called smtpd_per_record_deadline, is available with Postfix
    2.9-3.6. With older Postfix releases, the behavior is as if this parameter
    is set to "no".
This feature is available in Postfix 3.7 and later.
The default action when an SMTPD policy service request fails. Specify
  "DUNNO" to behave as if the failed SMTPD policy service request was
  not sent, and to continue processing other access restrictions, if any.
Limitations:
  - This parameter may specify any value that would be a valid SMTPD policy
      server response (or access(5) map lookup result). An
      access(5) map or policy server in this parameter value may need to
      be declared in advance with a restriction_class setting.
- If the specified action invokes another check_policy_service request, that
      request will have the built-in default action.
    
 
This feature is available in Postfix 3.0 and later.
The time after which an idle SMTPD policy service connection is closed.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
The time after which an active SMTPD policy service connection is closed.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
Optional information that the Postfix SMTP server specifies in the
  "policy_context" attribute of a policy service request (originally,
  to share the same service endpoint among multiple check_policy_service
  clients).
This feature is available in Postfix 3.1 and later.
The maximal number of requests per SMTPD policy service connection, or zero (no
  limit). Once a connection reaches this limit, the connection is closed and the
  next request will be sent over a new connection. This is a workaround to avoid
  error-recovery delays with policy servers that cannot maintain a persistent
  connection.
This feature is available in Postfix 3.0 and later.
The delay between attempts to resend a failed SMTPD policy service request.
  Specify a value greater than zero.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 3.0 and later.
The time limit for connecting to, writing to, or receiving from a delegated
  SMTPD policy server.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
The maximal number of attempts to send an SMTPD policy service request before
  giving up. Specify a value greater than zero.
This feature is available in Postfix 3.0 and later.
How the Postfix SMTP server announces itself to the proxy filter. By default,
  the Postfix hostname is used.
This feature is available in Postfix 2.1 and later.
The hostname and TCP port of the mail filtering proxy server. The proxy receives
  all mail from the Postfix SMTP server, and is supposed to give the result to
  another Postfix SMTP server process.
Specify "host:port" or "inet:host:port" for a
    TCP endpoint, or "unix:pathname" for a UNIX-domain endpoint. The
    host can be specified as an IP address or as a symbolic name; no MX lookups
    are done. When no "host" or "host:" is specified, the
    local machine is assumed. Pathname interpretation is relative to the Postfix
    queue directory.
This feature is available in Postfix 2.1 and later.
The "inet:" and "unix:" prefixes are available
    in Postfix 2.3 and later.
List of options that control how the Postfix SMTP server communicates with a
  before-queue content filter. Specify zero or more of the following, separated
  by comma or whitespace.
  - speed_adjust
- Do not connect to a before-queue content filter until an entire message
      has been received. This reduces the number of simultaneous before-queue
      content filter processes.
NOTE 1: A filter must not selectively reject recipients of
    a multi-recipient message. Rejecting all recipients is OK, as is accepting
    all recipients.
NOTE 2: This feature increases the minimum amount of free queue
    space by $message_size_limit. The extra space is needed to save the message
    to a temporary file.
  
This feature is available in Postfix 2.7 and later.
The time limit for connecting to a proxy filter and for sending or receiving
  information. When a connection fails the client gets a generic error message
  while more detailed information is logged to the maillog file.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.1 and later.
The maximal number of recipients that the Postfix SMTP server accepts per
  message delivery request.
The number of recipients that a remote SMTP client can send in excess of the
  limit specified with $smtpd_recipient_limit, before the Postfix SMTP server
  increments the per-session error count for each excess recipient.
Optional restrictions that the Postfix SMTP server applies in the context of a
  client RCPT TO command, after smtpd_relay_restrictions. See
  SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access
  restriction lists" for a discussion of evaluation context and time.
With Postfix versions before 2.10, the rules for relay permission
    and spam blocking were combined under smtpd_recipient_restrictions,
    resulting in error-prone configuration. As of Postfix 2.10, relay permission
    rules are preferably implemented with smtpd_relay_restrictions, so that a
    permissive spam blocking policy under smtpd_recipient_restrictions will no
    longer result in a permissive mail relay policy.
For backwards compatibility, sites that migrate from Postfix
    versions before 2.10 can set smtpd_relay_restrictions to the empty value,
    and use smtpd_recipient_restrictions exactly as before.
IMPORTANT: Either the smtpd_relay_restrictions or the
    smtpd_recipient_restrictions parameter must specify at least one of the
    following restrictions. Otherwise Postfix will refuse to receive mail:
  
reject, reject_unauth_destination
  
defer, defer_if_permit, defer_unauth_destination
Specify a list of restrictions, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace.
    Restrictions are applied in the order as specified; the first restriction
    that matches wins.
The following restrictions are specific to the recipient address
    that is received with the RCPT TO command.
  - check_recipient_access type:table
- Search the specified access(5) database for the resolved RCPT TO
      address, domain, parent domains, or localpart@, and execute the
      corresponding action.
    
 
- check_recipient_a_access type:table
- Search the specified access(5) database for the IP addresses for
      the RCPT TO domain, and execute the corresponding action. Note: a result
      of "OK" is not allowed for safety reasons. Instead, use DUNNO in
      order to exclude specific hosts from denylists. This feature is available
      in Postfix 3.0 and later.
    
 
- check_recipient_mx_access type:table
- Search the specified access(5) database for the MX hosts for the
      RCPT TO domain, and execute the corresponding action. If no MX record is
      found, look up A or AAAA records, just like the Postfix SMTP client would.
      Note: a result of "OK" is not allowed for safety reasons.
      Instead, use DUNNO in order to exclude specific hosts from denylists. This
      feature is available in Postfix 2.1 and later.
    
 
- check_recipient_ns_access type:table
- Search the specified access(5) database for the DNS servers for the
      RCPT TO domain, and execute the corresponding action. Note: a result of
      "OK" is not allowed for safety reasons. Instead, use DUNNO in
      order to exclude specific hosts from denylists. This feature is available
      in Postfix 2.1 and later.
    
 
- permit_auth_destination
- Permit the request when one of the following is true:
  - Postfix is a mail forwarder: the resolved RCPT TO domain matches
      $relay_domains or a subdomain thereof, and the address contains no
      sender-specified routing (user@elsewhere@domain),
- Postfix is the final destination: the resolved RCPT TO domain matches
      $mydestination, $inet_interfaces, $proxy_interfaces,
      $virtual_alias_domains, or $virtual_mailbox_domains, and the address
      contains no sender-specified routing (user@elsewhere@domain).
    
 
  - permit_mx_backup
- Permit the request when the local mail system is a backup MX for the RCPT
      TO domain, or when the domain is an authorized destination (see
      permit_auth_destination for definition).
  - Safety: permit_mx_backup does not accept addresses that have
      sender-specified routing information (example:
    user@elsewhere@domain).
- Safety: permit_mx_backup can be vulnerable to mis-use when access is not
      restricted with permit_mx_backup_networks.
- Safety: as of Postfix version 2.3, permit_mx_backup no longer accepts the
      address when the local mail system is a primary MX for the recipient
      domain. Exception: permit_mx_backup accepts the address when it specifies
      an authorized destination (see permit_auth_destination for
    definition).
- Limitation: mail may be rejected in case of a temporary DNS lookup problem
      with Postfix prior to version 2.0.
    
 
  - reject_non_fqdn_recipient
- Reject the request when the RCPT TO address specifies a domain that is not
      in fully-qualified domain form, as required by the RFC.
    
 The non_fqdn_reject_code parameter specifies the response code for rejected
      requests (default: 504).
 
- reject_rhsbl_recipient rbl_domain=d.d.d.d
- Reject the request when the RCPT TO domain is listed with the A record
      "d.d.d.d" under rbl_domain (Postfix version 2.1
      and later only). Each "d" is a number, or a pattern
      inside "[]" that contains one or more ";"-separated
      numbers or number..number ranges (Postfix version 2.8 and later). If no
      "=d.d.d.d" is specified, reject the request when the RCPT
      TO domain is listed with any A record under rbl_domain.
    
 The maps_rbl_reject_code parameter specifies the response code for rejected
      requests (default: 554); the default_rbl_reply parameter specifies the
      default server reply; and the rbl_reply_maps parameter specifies tables
      with server replies indexed by rbl_domain. This feature is
      available in Postfix version 2.0 and later.
 
- reject_unauth_destination
- Reject the request unless one of the following is true:
  - Postfix is a mail forwarder: the resolved RCPT TO domain matches
      $relay_domains or a subdomain thereof, and contains no sender-specified
      routing (user@elsewhere@domain),
- Postfix is the final destination: the resolved RCPT TO domain matches
      $mydestination, $inet_interfaces, $proxy_interfaces,
      $virtual_alias_domains, or $virtual_mailbox_domains, and contains no
      sender-specified routing (user@elsewhere@domain).
    
 The relay_domains_reject_code parameter specifies the response code for
      rejected requests (default: 554).
 
  - defer_unauth_destination
- Reject the same requests as reject_unauth_destination, with a
      non-permanent error code. This feature is available in Postfix 2.10 and
      later.
    
 
- reject_unknown_recipient_domain
- Reject the request when Postfix is not final destination for the recipient
      domain, and the RCPT TO domain has 1) no DNS MX and no DNS A record or 2)
      a malformed MX record such as a record with a zero-length MX hostname
      (Postfix version 2.3 and later).
    
 The reply is specified with the unknown_address_reject_code parameter
      (default: 450), unknown_address_tempfail_action (default:
      defer_if_permit), or 556 (nullmx, Postfix 3.0 and later). See the
      respective parameter descriptions for details.
 
- reject_unlisted_recipient (with Postfix version 2.0:
    check_recipient_maps)
- Reject the request when the RCPT TO address is not listed in the list of
      valid recipients for its domain class. See the
      smtpd_reject_unlisted_recipient parameter description for details. This
      feature is available in Postfix 2.1 and later.
    
 
- reject_unverified_recipient
- Reject the request when mail to the RCPT TO address is known to bounce, or
      when the recipient address destination is not reachable. Address
      verification information is managed by the verify(8) server; see
      the ADDRESS_VERIFICATION_README file for details.
    
 The unverified_recipient_reject_code parameter specifies the numerical
      response code when an address is known to bounce (default: 450, change it
      to 550 when you are confident that it is safe to do so).
 The unverified_recipient_defer_code parameter specifies the numerical
      response code when an address probe failed due to a temporary problem
      (default: 450).
 The unverified_recipient_tempfail_action parameter specifies the action
      after address probe failure due to a temporary problem (default:
      defer_if_permit).
 This feature breaks for aliased addresses with
      "enable_original_recipient = no" (Postfix <= 3.2).
 This feature is available in Postfix 2.1 and later.
 
Other restrictions that are valid in this context:
  - Generic restrictions that can be used in any SMTP command context,
      described under smtpd_client_restrictions.
- SMTP command specific restrictions described under
      smtpd_client_restrictions, smtpd_helo_restrictions and
      smtpd_sender_restrictions.
    
 
Example:
# The Postfix before 2.10 default mail relay policy. Later Postfix
# versions implement this preferably with smtpd_relay_restrictions.
smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination
Optional information that is appended after each Postfix SMTP server 4XX or 5XX
  response.
The following example uses "\c" at the start of the
    template (supported in Postfix 2.10 and later) to suppress the line break
    between the reply text and the footer text. With earlier Postfix versions,
    the footer text always begins on a new line, and the "\c" is
    output literally.
/etc/postfix/main.cf:
    smtpd_reject_footer = \c. For assistance, call 800-555-0101.
     Please provide the following information in your problem report:
     time ($localtime), client ($client_address) and server
     ($server_name).
Server response:
    550-5.5.1 <user@example> Recipient address rejected: User
    unknown. For assistance, call 800-555-0101. Please provide the
    following information in your problem report: time (Jan 4 15:42:00),
    client (192.168.1.248) and server (mail1.example.com).
Note: the above text is meant to make it easier to find the
    Postfix logfile records for a failed SMTP session. The text itself is not
    logged to the Postfix SMTP server's maillog file.
Be sure to keep the text as short as possible. Long text may be
    truncated before it is logged to the remote SMTP client's maillog file, or
    before it is returned to the sender in a delivery status notification.
The template text is not subject to Postfix configuration
    parameter $name expansion. Instead, this feature supports a limited number
    of $name attributes in the footer text. These attributes are replaced with
    their current value for the SMTP session.
Note: specify $$name in footer text that is looked up from regexp:
    or pcre:-based smtpd_reject_footer_maps, otherwise the Postfix server will
    not use the footer text and will log a warning instead.
  - client_address
- The Client IP address that is logged in the maillog file.
    
 
- client_port
- The client TCP port that is logged in the maillog file.
    
 
- localtime
- The server local time (Mmm dd hh:mm:ss) that is logged in the maillog
      file.
    
 
- server_name
- The server's myhostname value. This attribute is made available for sites
      with multiple MTAs (perhaps behind a load-balancer), where the server name
      can help the server support team to quickly find the right log files.
    
 
Notes:
  - NOT SUPPORTED are other attributes such as sender, recipient, or main.cf
      parameters.
- For safety reasons, text that does not match $smtpd_expansion_filter is
      censored.
    
 
This feature supports the two-character sequence \n as a request
    for a line break in the footer text. Postfix automatically inserts after
    each line break the three-digit SMTP reply code (and optional enhanced
    status code) from the original Postfix reject message.
To work around mail software that mis-handles multi-line replies,
    specify the two-character sequence \c at the start of the template. This
    suppresses the line break between the reply text and the footer text
    (Postfix 2.10 and later).
This feature is available in Postfix 2.8 and later.
Lookup tables, indexed by the complete Postfix SMTP server 4xx or 5xx response,
  with reject footer templates. See smtpd_reject_footer for details.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
This feature is available in Postfix 3.4 and later.
Request that the Postfix SMTP server rejects mail for unknown recipient
  addresses, even when no explicit reject_unlisted_recipient access restriction
  is specified. This prevents the Postfix queue from filling up with
  undeliverable MAILER-DAEMON messages.
An address is considered "unknown" when 1) it does not
    match a virtual(5) alias or canonical(5) mapping, and 2) the
    address is not valid for its address class. For a definition of class-based
    address validation, see ADDRESS_CLASS_README.
This feature is available in Postfix 2.1 and later.
Request that the Postfix SMTP server rejects mail from unknown sender addresses,
  even when no explicit reject_unlisted_sender access restriction is specified.
  This can slow down an explosion of forged mail from worms or viruses.
An address is considered "unknown" when 1) it does not
    match a virtual(5) alias or canonical(5) mapping, and 2) the
    address is not valid for its address class. For a definition of class-based
    address validation, see ADDRESS_CLASS_README.
This feature is available in Postfix 2.1 and later.
Evaluate smtpd_relay_restrictions before smtpd_recipient_restrictions.
  Historically, smtpd_relay_restrictions was evaluated after
  smtpd_recipient_restrictions, contradicting documented behavior.
Background: the smtpd_relay_restrictions feature is primarily
    designed to enforce a mail relaying policy, while
    smtpd_recipient_restrictions is primarily designed to enforce spam blocking
    policy. Both are evaluated while replying to the RCPT TO command, and both
    support the same features.
This feature is available in Postfix 3.6 and later.
Access restrictions for mail relay control that the Postfix SMTP server applies
  in the context of the RCPT TO command, before smtpd_recipient_restrictions.
  See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access
  restriction lists" for a discussion of evaluation context and time.
With Postfix versions before 2.10, the rules for relay permission
    and spam blocking were combined under smtpd_recipient_restrictions,
    resulting in error-prone configuration. As of Postfix 2.10, relay permission
    rules are preferably implemented with smtpd_relay_restrictions, so that a
    permissive spam blocking policy under smtpd_recipient_restrictions will no
    longer result in a permissive mail relay policy.
For backwards compatibility, sites that migrate from Postfix
    versions before 2.10 can set smtpd_relay_restrictions to the empty value,
    and use smtpd_recipient_restrictions exactly as before.
By default, the Postfix SMTP server accepts:
  - Mail from clients whose IP address matches $mynetworks, or:
- Mail from clients who are SASL authenticated, or:
- Mail to remote destinations that match $relay_domains, except for
      addresses that contain sender-specified routing (user@elsewhere@domain),
      or:
- Mail to local destinations that match $inet_interfaces or
      $proxy_interfaces, $mydestination, $virtual_alias_domains, or
      $virtual_mailbox_domains.
    
 
IMPORTANT: Either the smtpd_relay_restrictions or the
    smtpd_recipient_restrictions parameter must specify at least one of the
    following restrictions. Otherwise Postfix will refuse to receive mail:
  
reject, reject_unauth_destination
  
defer, defer_if_permit, defer_unauth_destination
Specify a list of restrictions, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace.
    The same restrictions are available as documented under
    smtpd_recipient_restrictions.
This feature is available in Postix 2.10 and later.
User-defined aliases for groups of access restrictions. The aliases can be
  specified in smtpd_recipient_restrictions etc., and on the right-hand side of
  a Postfix access(5) table.
One major application is for implementing per-recipient UCE
    control. See the RESTRICTION_CLASS_README document for other examples.
The application name that the Postfix SMTP server uses for SASL server
  initialization. This controls the name of the SASL configuration file. The
  default value is smtpd, corresponding to a SASL configuration file
  named smtpd.conf.
This feature is available in Postfix 2.1 and 2.2. With Postfix 2.3
    it was renamed to smtpd_sasl_path.
Enable SASL authentication in the Postfix SMTP server. By default, the Postfix
  SMTP server does not use authentication.
If a remote SMTP client is authenticated, the
    permit_sasl_authenticated access restriction can be used to permit relay
    access, like this:
  
# With Postfix 2.10 and later, the mail relay policy is
# preferably specified under smtpd_relay_restrictions.
smtpd_relay_restrictions =
    permit_mynetworks, permit_sasl_authenticated, ...
# With Postfix before 2.10, the relay policy can be
# specified only under smtpd_recipient_restrictions.
smtpd_recipient_restrictions =
    permit_mynetworks, permit_sasl_authenticated, ...
To reject all SMTP connections from unauthenticated clients,
    specify "smtpd_delay_reject = yes" (which is the default) and
  use:
  
smtpd_client_restrictions = permit_sasl_authenticated, reject
See the SASL_README file for SASL configuration and operation
    details.
Report the SASL authenticated user name in the smtpd(8) Received message
  header.
This feature is available in Postfix 2.3 and later.
What remote SMTP clients the Postfix SMTP server will not offer AUTH support to.
Some clients (Netscape 4 at least) have a bug that causes them to
    require a login and password whenever AUTH is offered, whether it's
    necessary or not. To work around this, specify, for example, $mynetworks to
    prevent Postfix from offering AUTH to local clients.
Specify a list of network/netmask patterns, separated by commas
    and/or whitespace. The mask specifies the number of bits in the network part
    of a host address. You can also specify "/file/name" or
    "type:table" patterns. A "/file/name" pattern is
    replaced by its contents; a "type:table" lookup table is matched
    when a table entry matches a lookup string (the lookup result is ignored).
    Continue long lines by starting the next line with whitespace. Specify
    "!pattern" to exclude an address or network block from the list.
    The form "!/file/name" is supported only in Postfix version 2.4
    and later.
Note: IP version 6 address information must be specified inside []
    in the smtpd_sasl_exceptions_networks value, and in files specified with
    "/file/name". IP version 6 addresses contain the ":"
    character, and would otherwise be confused with a "type:table"
    pattern.
Example:
smtpd_sasl_exceptions_networks = $mynetworks
This feature is available in Postfix 2.1 and later.
The name of the Postfix SMTP server's local SASL authentication realm.
By default, the local authentication realm name is the null
    string.
Examples:
smtpd_sasl_local_domain = $mydomain
smtpd_sasl_local_domain = $myhostname
If non-empty, a filter for the SASL mechanism names that the Postfix SMTP server
  will announce in the EHLO response. By default, the Postfix SMTP server will
  not announce the EXTERNAL mechanism, because Postfix support for that is not
  implemented.
Specify mechanism names, "/file/name" patterns, or
    "type:table" lookup tables, separated by comma or whitespace. The
    right-hand side result from "type:table" lookups is ignored.
    Specify "!pattern" to exclude a mechanism name from the list.
Examples:
smtpd_sasl_mechanism_filter = !external, !gssapi, static:rest
smtpd_sasl_mechanism_filter = login, plain
smtpd_sasl_mechanism_filter = /etc/postfix/smtpd_mechs
This feature is available in Postfix 3.6 and later.
Implementation-specific information that the Postfix SMTP server passes through
  to the SASL plug-in implementation that is selected with
  smtpd_sasl_type. Typically this specifies the name of a configuration
  file or rendezvous point.
This feature is available in Postfix 2.3 and later. In earlier
    releases it was called smtpd_sasl_application_name.
The maximum length of a SASL client's response to a server challenge. When the
  client's "initial response" is longer than the normal limit for SMTP
  commands, the client must omit its initial response, and wait for an empty
  server challenge; it can then send what would have been its "initial
  response" as a response to the empty server challenge. RFC4954 requires
  the server to accept client responses up to at least 12288 octets of
  base64-encoded text. The default value is therefore also the minimum value
  accepted for this parameter.
This feature is available in Postfix 3.4 and later. Prior versions
    use "line_length_limit", which may need to be raised to
    accommodate larger client responses, as may be needed with GSSAPI
    authentication of Windows AD users who are members of many groups.
Postfix SMTP server SASL security options; as of Postfix 2.3 the list of
  available features depends on the SASL server implementation that is selected
  with smtpd_sasl_type.
The following security features are defined for the cyrus
    server SASL implementation:
Restrict what authentication mechanisms the Postfix SMTP server
    will offer to the client. The list of available authentication mechanisms is
    system dependent.
Specify zero or more of the following:
  - noplaintext
- Disallow methods that use plaintext passwords.
    
 
- noactive
- Disallow methods subject to active (non-dictionary) attack.
    
 
- nodictionary
- Disallow methods subject to passive (dictionary) attack.
    
 
- noanonymous
- Disallow methods that allow anonymous authentication.
    
 
- forward_secrecy
- Only allow methods that support forward secrecy (Dovecot only).
    
 
- mutual_auth
- Only allow methods that provide mutual authentication (not available with
      Cyrus SASL version 1).
    
 
By default, the Postfix SMTP server accepts plaintext passwords
    but not anonymous logins.
Warning: it appears that clients try authentication methods in the
    order as advertised by the server (e.g., PLAIN ANONYMOUS CRAM-MD5) which
    means that if you disable plaintext passwords, clients will log in
    anonymously, even when they should be able to use CRAM-MD5. So, if you
    disable plaintext logins, disable anonymous logins too. Postfix treats
    anonymous login as no authentication.
Example:
smtpd_sasl_security_options = noanonymous, noplaintext
The service name that is passed to the SASL plug-in that is selected with
  smtpd_sasl_type and smtpd_sasl_path.
This feature is available in Postfix 2.11 and later. Prior
    versions behave as if "smtp" is specified.
The SASL authentication security options that the Postfix SMTP server uses for
  TLS encrypted SMTP sessions.
This feature is available in Postfix 2.2 and later.
The SASL plug-in type that the Postfix SMTP server should use for
  authentication. The available types are listed with the "postconf
  -a" command.
This feature is available in Postfix 2.3 and later.
Optional lookup table with the SASL login names that own the sender (MAIL FROM)
  addresses.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found. With lookups from indexed files such as DB or
    DBM, or from networked tables such as NIS, LDAP or SQL, the following search
    operations are done with a sender address of user@domain:
  - 1) user@domain
- This table lookup is always done and has the highest precedence.
    
 
- 2) user
- This table lookup is done only when the domain part of the sender
      address matches $myorigin, $mydestination, $inet_interfaces or
      $proxy_interfaces.
    
 
- 3) @domain
- This table lookup is done last and has the lowest precedence.
    
 
In all cases the result of table lookup must be either "not
    found" or a list of SASL login names separated by comma and/or
    whitespace.
Optional restrictions that the Postfix SMTP server applies in the context of a
  client MAIL FROM command. See SMTPD_ACCESS_README, section "Delayed
  evaluation of SMTP access restriction lists" for a discussion of
  evaluation context and time.
The default is to permit everything.
Specify a list of restrictions, separated by commas and/or
    whitespace. Continue long lines by starting the next line with whitespace.
    Restrictions are applied in the order as specified; the first restriction
    that matches wins.
The following restrictions are specific to the sender address
    received with the MAIL FROM command.
  - check_sender_access type:table
- Search the specified access(5) database for the MAIL FROM address,
      domain, parent domains, or localpart@, and execute the corresponding
      action.
    
 
- check_sender_a_access type:table
- Search the specified access(5) database for the IP addresses for
      the MAIL FROM domain, and execute the corresponding action. Note: a result
      of "OK" is not allowed for safety reasons. Instead, use DUNNO in
      order to exclude specific hosts from denylists. This feature is available
      in Postfix 3.0 and later.
    
 
- check_sender_mx_access type:table
- Search the specified access(5) database for the MX hosts for the
      MAIL FROM domain, and execute the corresponding action. If no MX record is
      found, look up A or AAAA records, just like the Postfix SMTP client would.
      Note: a result of "OK" is not allowed for safety reasons.
      Instead, use DUNNO in order to exclude specific hosts from denylists. This
      feature is available in Postfix 2.1 and later.
    
 
- check_sender_ns_access type:table
- Search the specified access(5) database for the DNS servers for the
      MAIL FROM domain, and execute the corresponding action. Note: a result of
      "OK" is not allowed for safety reasons. Instead, use DUNNO in
      order to exclude specific hosts from denylists. This feature is available
      in Postfix 2.1 and later.
    
 
- reject_authenticated_sender_login_mismatch
- Reject the request when the client is authenticated with SASL, but either
      the MAIL FROM address is not listed in $smtpd_sender_login_maps, or the
      SASL login name is not an owner for that address.
    
 This prevents an authenticated client from using a MAIL FROM address that
      they do not explicitly own.
 This feature is available in Postfix version 2.1 and later.
 
- reject_known_sender_login_mismatch
- When the client is authenticated with SASL, reject the request when the
      MAIL FROM address is listed in $smtpd_sender_login_maps, but the SASL
      login name is not an owner for that address.
    
 When the client is not authenticated with SASL, reject the request when SASL
      is enabled, and the MAIL FROM address is listed in
      $smtpd_sender_login_maps.
 This protects any MAIL FROM address that is listed in
      $smtpd_sender_login_maps, while still allowing a client to use any
      unlisted MAIL FROM address.
 This feature is available in Postfix version 2.11 and later.
 
- reject_non_fqdn_sender
- Reject the request when the MAIL FROM address specifies a domain that is
      not in fully-qualified domain form as required by the RFC.
    
 The non_fqdn_reject_code parameter specifies the response code for rejected
      requests (default: 504).
 
- reject_rhsbl_sender rbl_domain=d.d.d.d
- Reject the request when the MAIL FROM domain is listed with the A record
      "d.d.d.d" under rbl_domain (Postfix version 2.1
      and later only). Each "d" is a number, or a pattern
      inside "[]" that contains one or more ";"-separated
      numbers or number..number ranges (Postfix version 2.8 and later). If no
      "=d.d.d.d" is specified, reject the request when the MAIL
      FROM domain is listed with any A record under rbl_domain.
    
 The maps_rbl_reject_code parameter specifies the response code for rejected
      requests (default: 554); the default_rbl_reply parameter specifies the
      default server reply; and the rbl_reply_maps parameter specifies tables
      with server replies indexed by rbl_domain. This feature is
      available in Postfix 2.0 and later.
 
- reject_sender_login_mismatch
- As of Postfix 2.1, this is an alias for
      "reject_authenticated_sender_login_mismatch,
      reject_unauthenticated_sender_login_mismatch".
    
 
- reject_unauthenticated_sender_login_mismatch
- Reject the request when SASL is enabled, the MAIL FROM address is listed
      in $smtpd_sender_login_maps, but the client is not authenticated with
      SASL.
    
 With SASL enabled, this prevents an unauthenticated client from using any
      MAIL FROM address that is listed in $smtpd_sender_login_maps.
 This feature is available in Postfix version 2.1 and later.
 
- reject_unknown_sender_domain
- Reject the request when Postfix is not the final destination for the
      sender address, and the MAIL FROM domain has 1) no DNS MX and no DNS A
      record, or 2) a malformed MX record such as a record with a zero-length MX
      hostname (Postfix version 2.3 and later).
    
 The reply is specified with the unknown_address_reject_code parameter
      (default: 450), unknown_address_tempfail_action (default:
      defer_if_permit), or 550 (nullmx, Postfix 3.0 and later). See the
      respective parameter descriptions for details.
 
- reject_unlisted_sender
- Reject the request when the MAIL FROM address is not listed in the list of
      valid recipients for its domain class. See the
      smtpd_reject_unlisted_sender parameter description for details. This
      feature is available in Postfix 2.1 and later.
    
 
- reject_unverified_sender
- Reject the request when mail to the MAIL FROM address is known to bounce,
      or when the sender address destination is not reachable. Address
      verification information is managed by the verify(8) server; see
      the ADDRESS_VERIFICATION_README file for details.
    
 The unverified_sender_reject_code parameter specifies the numerical response
      code when an address is known to bounce (default: 450, change into 550
      when you are confident that it is safe to do so).
 The unverified_sender_defer_code specifies the numerical response code when
      an address probe failed due to a temporary problem (default: 450).
 The unverified_sender_tempfail_action parameter specifies the action after
      address probe failure due to a temporary problem (default:
      defer_if_permit).
 This feature breaks for aliased addresses with
      "enable_original_recipient = no" (Postfix <= 3.2).
 This feature is available in Postfix 2.1 and later.
 
Other restrictions that are valid in this context:
  - Generic restrictions that can be used in any SMTP command context,
      described under smtpd_client_restrictions.
- SMTP command specific restrictions described under
      smtpd_client_restrictions and smtpd_helo_restrictions.
- SMTP command specific restrictions described under
      smtpd_recipient_restrictions. When recipient restrictions are listed under
      smtpd_sender_restrictions, they have effect only with
      "smtpd_delay_reject = yes", so that $smtpd_sender_restrictions
      is evaluated at the time of the RCPT TO command.
    
 
Examples:
smtpd_sender_restrictions = reject_unknown_sender_domain
smtpd_sender_restrictions = reject_unknown_sender_domain,
    check_sender_access hash:/etc/postfix/access
The internal service that postscreen(8) hands off allowed connections to.
  In a future version there may be different classes of SMTP service.
This feature is available in Postfix 2.8.
The number of errors a remote SMTP client is allowed to make without delivering
  mail before the Postfix SMTP server slows down all its responses.
  - With Postfix version 2.1 and later, when the error count is >
      $smtpd_soft_error_limit, the Postfix SMTP server delays all responses by
      $smtpd_error_sleep_time.
- With Postfix versions 2.0 and earlier, when the error count is >
      $smtpd_soft_error_limit, the Postfix SMTP server delays all responses by
      the larger of (number of errors) seconds or $smtpd_error_sleep_time.
- With Postfix versions 2.0 and earlier, when the error count is <=
      $smtpd_soft_error_limit, the Postfix SMTP server delays 4XX and 5XX
      responses by $smtpd_error_sleep_time.
    
 
The time limit for Postfix SMTP server write and read operations during TLS
  startup and shutdown handshake procedures. The current default value is
  stress-dependent. Before Postfix version 2.8, it was fixed at 300s.Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.2 and later.
When the Postfix SMTP server wants to send an SMTP server response, how long the
  Postfix SMTP server will wait for an underlying network write operation to
  complete; and when the Postfix SMTP server Postfix wants to receive an SMTP
  client request, how long the Postfix SMTP server will wait for an underlying
  network read operation to complete. See the smtpd_per_request_deadline for how
  this time limit may be enforced (with Postfix 2.9-3.6 see
  smtpd_per_record_deadline).
Normally the default limit is 300s, but it changes under overload
    to just 10s. With Postfix 2.5 and earlier, the SMTP server always uses a
    time limit of 300s by default.
Note: if you set SMTP time limits to very large values you may
    have to update the global ipc_timeout parameter.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
A file containing (PEM format) CA certificates of root CAs trusted to sign
  either remote SMTP client certificates or intermediate CA certificates. These
  are loaded into memory before the smtpd(8) server enters the chroot
  jail. If the number of trusted roots is large, consider using smtpd_tls_CApath
  instead, but note that the latter directory must be present in the chroot jail
  if the smtpd(8) server is chrooted. This file may also be used to
  augment the server certificate trust chain, but it is best to include all the
  required certificates directly in the server certificate file.
Specify "smtpd_tls_CAfile = /path/to/system_CA_file" to
    use ONLY the system-supplied default Certification Authority
  certificates.
Specify "tls_append_default_CA = no" to prevent Postfix
    from appending the system-supplied default CAs and trusting third-party
    certificates.
By default (see smtpd_tls_ask_ccert), client certificates are not
    requested, and smtpd_tls_CAfile should remain empty. If you do make use of
    client certificates, the distinguished names (DNs) of the Certification
    Authorities listed in smtpd_tls_CAfile are sent to the remote SMTP client in
    the client certificate request message. MUAs with multiple client
    certificates may use the list of preferred Certification Authorities to
    select the correct client certificate. You may want to put your
    "preferred" CA or CAs in this file, and install other trusted CAs
    in $smtpd_tls_CApath.
Example:
smtpd_tls_CAfile = /etc/postfix/CAcert.pem
This feature is available in Postfix 2.2 and later.
A directory containing (PEM format) CA certificates of root CAs trusted to sign
  either remote SMTP client certificates or intermediate CA certificates. Do not
  forget to create the necessary "hash" links with, for example,
  "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs". To use
  smtpd_tls_CApath in chroot mode, this directory (or a copy) must be inside the
  chroot jail.
Specify "smtpd_tls_CApath =
    /path/to/system_CA_directory" to use ONLY the system-supplied default
    Certification Authority certificates.
Specify "tls_append_default_CA = no" to prevent Postfix
    from appending the system-supplied default CAs and trusting third-party
    certificates.
By default (see smtpd_tls_ask_ccert), client certificates are not
    requested, and smtpd_tls_CApath should remain empty. In contrast to
    smtpd_tls_CAfile, DNs of Certification Authorities installed in
    $smtpd_tls_CApath are not included in the client certificate request
    message. MUAs with multiple client certificates may use the list of
    preferred Certification Authorities to select the correct client
    certificate. You may want to put your "preferred" CA or CAs in
    $smtpd_tls_CAfile, and install the remaining trusted CAs in
    $smtpd_tls_CApath.
Example:
smtpd_tls_CApath = /etc/postfix/certs
This feature is available in Postfix 2.2 and later.
Force the Postfix SMTP server to issue a TLS session id, even when TLS session
  caching is turned off (smtpd_tls_session_cache_database is empty). This
  behavior is compatible with Postfix < 2.3.
With Postfix 2.3 and later the Postfix SMTP server can disable
    session id generation when TLS session caching is turned off. This keeps
    remote SMTP clients from caching sessions that almost certainly cannot be
    re-used.
By default, the Postfix SMTP server always generates TLS session
    ids. This works around a known defect in mail client applications such as MS
    Outlook, and may also prevent interoperability issues with other MTAs.
Example:
smtpd_tls_always_issue_session_ids = no
This feature is available in Postfix 2.3 and later.
Ask a remote SMTP client for a client certificate. This information is needed
  for certificate based mail relaying with, for example, the
  permit_tls_clientcerts feature.
Some clients such as Netscape will either complain if no
    certificate is available (for the list of CAs in $smtpd_tls_CAfile) or will
    offer multiple client certificates to choose from. This may be annoying, so
    this option is "off" by default.
This feature is available in Postfix 2.2 and later.
When TLS encryption is optional in the Postfix SMTP server, do not announce or
  accept SASL authentication over unencrypted connections.
This feature is available in Postfix 2.2 and later.
The verification depth for remote SMTP client certificates. A depth of 1 is
  sufficient if the issuing CA is listed in a local CA file.
The default verification depth is 9 (the OpenSSL default) for
    compatibility with earlier Postfix behavior. Prior to Postfix 2.5, the
    default value was 5, but the limit was not actually enforced. If you have
    set this to a lower non-default value, certificates with longer trust chains
    may now fail to verify. Certificate chains with 1 or 2 CAs are common,
    deeper chains are more rare and any number between 5 and 9 should suffice in
    practice. You can choose a lower number if, for example, you trust
    certificates directly signed by an issuing CA but not any CAs it delegates
    to.
This feature is available in Postfix 2.2 and later.
File with the Postfix SMTP server RSA certificate in PEM format. This file may
  also contain the Postfix SMTP server private RSA key. With Postfix >= 3.4
  the preferred way to configure server keys and certificates is via the
  "smtpd_tls_chain_files" parameter.
Public Internet MX hosts without certificates signed by a
    "reputable" CA must generate, and be prepared to present to most
    clients, a self-signed or private-CA signed certificate. The client will not
    be able to authenticate the server, but unless it is running Postfix 2.3 or
    similar software, it will still insist on a server certificate.
For servers that are not public Internet MX hosts, Postfix
    supports configurations with no certificates. This entails the use of just
    the anonymous TLS ciphers, which are not supported by typical SMTP clients.
    Since some clients may not fall back to plain text after a TLS handshake
    failure, a certificate-less Postfix SMTP server will be unable to receive
    email from some TLS-enabled clients. To avoid accidental configurations with
    no certificates, Postfix enables certificate-less operation only when the
    administrator explicitly sets "smtpd_tls_cert_file = none". This
    ensures that new Postfix SMTP server configurations will not accidentally
    enable TLS without certificates.
Note that server certificates are not optional in TLS 1.3. To run
    without certificates you'd have to disable the TLS 1.3 protocol by including
    '!TLSv1.3' in "smtpd_tls_protocols" and perhaps also
    "smtpd_tls_mandatory_protocols". It is simpler instead to just
    configure a certificate chain. Certificate-less operation is not
    recommended.
Both RSA and DSA certificates are supported. When both types are
    present, the cipher used determines which certificate will be presented to
    the client. For Netscape and OpenSSL clients without special cipher choices
    the RSA certificate is preferred.
To enable a remote SMTP client to verify the Postfix SMTP server
    certificate, the issuing CA certificates must be made available to the
    client. You should include the required certificates in the server
    certificate file, the server certificate first, then the issuing CA(s)
    (bottom-up order).
Example: the certificate for "server.example.com" was
    issued by "intermediate CA" which itself has a certificate of
    "root CA". Create the server.pem file with "cat
    server_cert.pem intermediate_CA.pem root_CA.pem > server.pem".
If you also want to verify client certificates issued by these
    CAs, you can add the CA certificates to the smtpd_tls_CAfile, in which case
    it is not necessary to have them in the smtpd_tls_cert_file,
    smtpd_tls_dcert_file (obsolete) or smtpd_tls_eccert_file.
A certificate supplied here must be usable as an SSL server
    certificate and hence pass the "openssl verify -purpose sslserver
    ..." test.
Example:
smtpd_tls_cert_file = /etc/postfix/server.pem
This feature is available in Postfix 2.2 and later.
List of one or more PEM files, each holding one or more private keys directly
  followed by a corresponding certificate chain. The file names are separated by
  commas and/or whitespace. This parameter obsoletes the legacy
  algorithm-specific key and certificate file settings. When this parameter is
  non-empty, the legacy parameters are ignored, and a warning is logged if any
  are also non-empty.
With the proliferation of multiple private key algorithms-which,
    as of OpenSSL 1.1.1, include DSA (obsolete), RSA, ECDSA, Ed25519 and
    Ed448-it is increasingly impractical to use separate parameters to configure
    the key and certificate chain for each algorithm. Therefore, Postfix now
    supports storing multiple keys and corresponding certificate chains in a
    single file or in a set of files.
Each key must appear immediately before the corresponding
    certificate, optionally followed by additional issuer certificates that
    complete the certificate chain for that key. When multiple files are
    specified, they are equivalent to a single file that is concatenated from
    those files in the given order. Thus, while a key must always precede its
    certificate and issuer chain, it can be in a separate file, so long as that
    file is listed immediately before the file that holds the corresponding
    certificate chain. Once all the files are concatenated, the sequence of PEM
    objects must be: key1, cert1, [chain1], key2, cert2, [chain2],
    ..., keyN, certN, [chainN].
Storing the private key in the same file as the corresponding
    certificate is more reliable. With the key and certificate in separate
    files, there is a chance that during key rollover a Postfix process might
    load a private key and certificate from separate files that don't match.
    Various operational errors may even result in a persistent broken
    configuration in which the certificate does not match the private key.
The file or files must contain at most one key of each type. If,
    for example, two or more RSA keys and corresponding chains are listed,
    depending on the version of OpenSSL either only the last one will be used or
    a configuration error may be detected. Note that while "Ed25519"
    and "Ed448" are considered separate algorithms, the various ECDSA
    curves (typically one of prime256v1, secp384r1 or secp521r1) are considered
    as different parameters of a single "ECDSA" algorithm, so it is
    not presently possible to configure keys for more than one ECDSA curve.
RSA is still the most widely supported algorithm. Presently (late
    2018), ECDSA support is common, but not yet universal, and Ed25519 and Ed448
    support is mostly absent. Therefore, an RSA key should generally be
    configured, along with any additional keys for the other algorithms when
    desired.
Example (separate files for each key and corresponding certificate
    chain):
  
/etc/postfix/main.cf:
    smtpd_tls_chain_files =
        ${config_directory}/ed25519.pem,
        ${config_directory}/ed448.pem,
        ${config_directory}/rsa.pem
  
/etc/postfix/ed25519.pem:
    -----BEGIN PRIVATE KEY-----
    MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
    ...
    nC0egv51YPDWxEHom4QA
    -----END CERTIFICATE-----
  
/etc/postfix/ed448.pem:
    -----BEGIN PRIVATE KEY-----
    MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
    LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
    ...
    pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
    -----END CERTIFICATE-----
  
/etc/postfix/rsa.pem:
    -----BEGIN PRIVATE KEY-----
    MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
    ...
    ahQkZ3+krcaJvDSMgvu0tDc=
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
    ...
    Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
    -----END CERTIFICATE-----
Example (all keys and certificates in a single file):
  
/etc/postfix/main.cf:
    smtpd_tls_chain_files = ${config_directory}/chains.pem
  
/etc/postfix/chains.pem:
    -----BEGIN PRIVATE KEY-----
    MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
    ...
    nC0egv51YPDWxEHom4QA
    -----END CERTIFICATE-----
    -----BEGIN PRIVATE KEY-----
    MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
    LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
    ...
    pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
    -----END CERTIFICATE-----
    -----BEGIN PRIVATE KEY-----
    MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
    ...
    ahQkZ3+krcaJvDSMgvu0tDc=
    -----END PRIVATE KEY-----
    -----BEGIN CERTIFICATE-----
    MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
    ...
    Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
    -----END CERTIFICATE-----
This feature is available in Postfix 3.4 and later.
Obsolete Postfix < 2.3 control for the Postfix SMTP server TLS cipher list.
  It is easy to create interoperability problems by choosing a non-default
  cipher list. Do not use a non-default TLS cipherlist for MX hosts on the
  public Internet. Clients that begin the TLS handshake, but are unable to agree
  on a common cipher, may not be able to send any email to the SMTP server.
  Using a restricted cipher list may be more appropriate for a dedicated MSA or
  an internal mailhub, where one can exert some control over the TLS software
  and settings of the connecting clients.
Note: do not use "" quotes around the parameter
    value.
This feature is available with Postfix version 2.2. It is not used
    with Postfix 2.3 and later; use smtpd_tls_mandatory_ciphers instead.
The minimum TLS cipher grade that the Postfix SMTP server will use with
  opportunistic TLS encryption. Cipher types listed in smtpd_tls_exclude_ciphers
  are excluded from the base definition of the selected cipher grade. The
  default value is "medium" for Postfix releases after the middle of
  2015, "export" for older releases.
When TLS is mandatory the cipher grade is chosen via the
    smtpd_tls_mandatory_ciphers configuration parameter, see there for syntax
    details.
This feature is available in Postfix 2.6 and later. With earlier
    Postfix releases only the smtpd_tls_mandatory_ciphers parameter is
    implemented, and opportunistic TLS always uses "export" or better
    (i.e. all) ciphers.
File with the Postfix SMTP server DSA certificate in PEM format. This file may
  also contain the Postfix SMTP server private DSA key. The DSA algorithm is
  obsolete and should not be used.
See the discussion under smtpd_tls_cert_file for more details.
Example:
smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem
This feature is available in Postfix 2.2 and later.
File with DH parameters that the Postfix SMTP server should use with non-export
  EDH ciphers.
With Postfix >= 3.7, built with OpenSSL version is 3.0.0 or
    later, if the parameter value is either empty or "auto",
    then the DH parameter selection is delegated to the OpenSSL library, which
    selects appropriate parameters based on the TLS handshake. This choice is
    likely to be the most interoperable with SMTP clients using various TLS
    libraries, and custom local parameters are no longer recommended when using
    Postfix >= 3.7 built against OpenSSL 3.0.0.
The best-practice choice of parameters uses a 2048-bit prime. This
    is fine, despite the historical "1024" in the parameter name. Do
    not be tempted to use much larger values, performance degrades quickly, and
    you may also cease to interoperate with some mainstream SMTP clients. As of
    Postfix 3.1, the compiled-in default prime is 2048-bits, and it is not
    strictly necessary, though perhaps somewhat beneficial to generate custom DH
    parameters.
Instead of using the exact same parameter sets as distributed with
    other TLS packages, it is more secure to generate your own set of parameters
    with something like the following commands:
  
openssl dhparam -out /etc/postfix/dh2048.pem 2048
openssl dhparam -out /etc/postfix/dh1024.pem 1024
# As of Postfix 3.6, export-grade 512-bit DH parameters are no longer
# supported or needed.
openssl dhparam -out /etc/postfix/dh512.pem 512
It is safe to share the same DH parameters between multiple
    Postfix instances. If you prefer, you can generate separate parameters for
    each instance.
If you want to take maximal advantage of ciphers that offer
    forward secrecy see the Getting started section of FORWARD_SECRECY_README.
    The full document conveniently presents all information about Postfix
    "perfect" forward secrecy support in one place: what forward
    secrecy is, how to tweak settings, and what you can expect to see when
    Postfix uses ciphers with forward secrecy.
Example:
smtpd_tls_dh1024_param_file = /etc/postfix/dh2048.pem
This feature is available in Postfix 2.2 and later.
File with DH parameters that the Postfix SMTP server should use with
  export-grade EDH ciphers. The default SMTP server cipher grade is
  "medium" with Postfix releases after the middle of 2015, and as a
  result export-grade cipher suites are by default not used.
With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is
    no longer supported, and this parameter is silently ignored.
See also the discussion under the smtpd_tls_dh1024_param_file
    configuration parameter.
Example:
smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem
This feature is available in Postfix 2.2 and later, but is ignored
    in Postfix 3.6 and later.
File with the Postfix SMTP server DSA private key in PEM format. This file may
  be combined with the Postfix SMTP server DSA certificate file specified with
  $smtpd_tls_dcert_file. The DSA algorithm is obsolete and should not be used.
The private key must be accessible without a pass-phrase, i.e. it
    must not be encrypted. File permissions should grant read-only access to the
    system superuser account ("root"), and no access to anyone
  else.
This feature is available in Postfix 2.2 and later.
File with the Postfix SMTP server ECDSA certificate in PEM format. This file may
  also contain the Postfix SMTP server private ECDSA key. With Postfix >= 3.4
  the preferred way to configure server keys and certificates is via the
  "smtpd_tls_chain_files" parameter.
See the discussion under smtpd_tls_cert_file for more details.
Example:
smtpd_tls_eccert_file = /etc/postfix/ecdsa-scert.pem
This feature is available in Postfix 2.6 and later, when Postfix
    is compiled and linked with OpenSSL 1.0.0 or later.
File with the Postfix SMTP server ECDSA private key in PEM format. This file may
  be combined with the Postfix SMTP server ECDSA certificate file specified with
  $smtpd_tls_eccert_file. With Postfix >= 3.4 the preferred way to configure
  server keys and certificates is via the "smtpd_tls_chain_files"
  parameter.
The private key must be accessible without a pass-phrase, i.e. it
    must not be encrypted. File permissions should grant read-only access to the
    system superuser account ("root"), and no access to anyone
  else.
This feature is available in Postfix 2.6 and later, when Postfix
    is compiled and linked with OpenSSL 1.0.0 or later.
The Postfix SMTP server security grade for ephemeral elliptic-curve
  Diffie-Hellman (EECDH) key exchange. As of Postfix 3.6, the value of this
  parameter is always ignored, and Postfix behaves as though the auto
  value (described below) was chosen.
The available choices are:
  - auto
- Use the most preferred curve that is supported by both the client and the
      server. This setting requires Postfix >= 3.2 compiled and linked with
      OpenSSL >= 1.0.2. This is the default setting under the above
      conditions (and the only setting used with Postfix >= 3.6).
    
 
- none
- Don't use EECDH. Ciphers based on EECDH key exchange will be disabled.
      This is the default in Postfix versions 2.6 and 2.7.
    
 
- strong
- Use EECDH with approximately 128 bits of security at a reasonable
      computational cost. This is the default in Postfix versions 2.8-3.5.
    
 
- ultra
- Use EECDH with approximately 192 bits of security at computational cost
      that is approximately twice as high as 128 bit strength ECC.
    
 
If you want to take maximal advantage of ciphers that offer
    forward secrecy see the Getting started section of FORWARD_SECRECY_README.
    The full document conveniently presents all information about Postfix
    "perfect" forward secrecy support in one place: what forward
    secrecy is, how to tweak settings, and what you can expect to see when
    Postfix uses ciphers with forward secrecy.
This feature is available in Postfix 2.6 and later, when it is
    compiled and linked with OpenSSL 1.0.0 or later on platforms where EC
    algorithms have not been disabled by the vendor.
List of ciphers or cipher types to exclude from the SMTP server cipher list at
  all TLS security levels. Excluding valid ciphers can create interoperability
  problems. DO NOT exclude ciphers unless it is essential to do so. This is not
  an OpenSSL cipherlist; it is a simple list separated by whitespace and/or
  commas. The elements are a single cipher, or one or more "+"
  separated cipher properties, in which case only ciphers matching all
  the properties are excluded.
Examples (some of these will cause problems):
  
smtpd_tls_exclude_ciphers = aNULL
smtpd_tls_exclude_ciphers = MD5, DES
smtpd_tls_exclude_ciphers = DES+MD5
smtpd_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
smtpd_tls_exclude_ciphers = kEDH+aRSA
The first setting disables anonymous ciphers. The next setting
    disables ciphers that use the MD5 digest algorithm or the (single) DES
    encryption algorithm. The next setting disables ciphers that use MD5 and DES
    together. The next setting disables the two ciphers "AES256-SHA"
    and "DES-CBC3-MD5". The last setting disables ciphers that use
    "EDH" key exchange with RSA authentication.
This feature is available in Postfix 2.3 and later.
The message digest algorithm to construct remote SMTP client-certificate
  fingerprints or public key fingerprints (Postfix 2.9 and later) for
  check_ccert_access and permit_tls_clientcerts.
The default algorithm is sha256 with Postfix >= 3.6 and
    the compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
    the default algorithm is md5.
The best-practice algorithm is now sha256. Recent advances
    in hash function cryptanalysis have led to md5 and sha1 being deprecated in
    favor of sha256. However, as long as there are no known "second
    pre-image" attacks against the older algorithms, their use in this
    context, though not recommended, is still likely safe.
While additional digest algorithms are often available with
    OpenSSL's libcrypto, only those used by libssl in SSL cipher suites are
    available to Postfix. You'll likely find support for md5, sha1, sha256 and
    sha512.
To find the fingerprint of a specific certificate file, with a
    specific digest algorithm, run:
  
$ openssl x509 -noout -fingerprint -digest -in certfile.pem
The text to the right of "=" sign is the desired
    fingerprint. For example:
  
$ openssl x509 -noout -fingerprint -sha256 -in cert.pem
SHA256 Fingerprint=D4:6A:AB:19:24:...:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A
To extract the public key fingerprint from an X.509 certificate,
    you need to extract the public key from the certificate and compute the
    appropriate digest of its DER (ASN.1) encoding. With OpenSSL the
    "-pubkey" option of the "x509" command extracts the
    public key always in "PEM" format. We pipe the result to another
    OpenSSL command that converts the key to DER and then to the
    "dgst" command to compute the fingerprint.
Example:
  
$ openssl x509 -in cert.pem -noout -pubkey |
    openssl pkey -pubin -outform DER |
    openssl dgst -sha256 -c
(stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58
The Postfix SMTP server and client log the peer (leaf) certificate
    fingerprint and public key fingerprint when the TLS loglevel is 2 or
  higher.
Example: client-certificate access table, with sha256
    fingerprints:
  
/etc/postfix/main.cf:
    smtpd_tls_fingerprint_digest = sha256
    smtpd_client_restrictions =
        check_ccert_access hash:/etc/postfix/access,
        reject
/etc/postfix/access:
    # Action folded to next line...
    AF:88:7C:AD:51:95:6F:36:96:...:01:FB:2E:48:CD:AB:49:25:A2:3B
        OK
    85:16:78:FD:73:6E:CE:70:E0:...:5F:0D:3C:C8:6D:C4:2C:24:59:E1
        permit_auth_destination
This feature is available in Postfix 2.5 and later.
File with the Postfix SMTP server RSA private key in PEM format. This file may
  be combined with the Postfix SMTP server RSA certificate file specified with
  $smtpd_tls_cert_file. With Postfix >= 3.4 the preferred way to configure
  server keys and certificates is via the "smtpd_tls_chain_files"
  parameter.
The private key must be accessible without a pass-phrase, i.e. it
    must not be encrypted. File permissions should grant read-only access to the
    system superuser account ("root"), and no access to anyone
  else.
Enable additional Postfix SMTP server logging of TLS activity. Each logging
  level also includes the information that is logged at a lower logging level.
  - 
  
- 0 Disable logging of TLS activity.
    
 
- 
  
- 1 Log only a summary message on TLS handshake completion - no logging of
      client certificate trust-chain verification errors if client certificate
      verification is not required. With Postfix 2.8 and earlier, log the
      summary message, peer certificate summary information and unconditionally
      log trust-chain verification errors.
    
 
- 
  
- 2 Also log levels during TLS negotiation.
    
 
- 
  
- 3 Also log hexadecimal and ASCII dump of TLS negotiation process.
    
 
- 
  
- 4 Also log hexadecimal and ASCII dump of complete transmission after
      STARTTLS.
    
 
Do not use "smtpd_tls_loglevel = 2" or higher except in
    case of problems. Use of loglevel 4 is strongly discouraged.
This feature is available in Postfix 2.2 and later.
The minimum TLS cipher grade that the Postfix SMTP server will use with
  mandatory TLS encryption. The default grade ("medium") is
  sufficiently strong that any benefit from globally restricting TLS sessions to
  a more stringent grade is likely negligible, especially given the fact that
  many implementations still do not offer any stronger ("high" grade)
  ciphers, while those that do, will always use "high" grade ciphers.
  So insisting on "high" grade ciphers is generally
  counter-productive. Allowing "export" or "low" ciphers is
  typically not a good idea, as systems limited to just these are limited to
  obsolete browsers. No known SMTP clients fail to support at least one
  "medium" or "high" grade cipher.
The following cipher grades are supported:
  - high
- Enable only "HIGH" grade OpenSSL ciphers. The underlying
      cipherlist is specified via the tls_high_cipherlist configuration
      parameter, which you are strongly encouraged to not change.
    
 
- medium
- Enable "MEDIUM" grade or stronger OpenSSL ciphers. These use
      128-bit or longer symmetric bulk-encryption keys. This is the default
      minimum strength for mandatory TLS encryption. The underlying cipherlist
      is specified via the tls_medium_cipherlist configuration parameter, which
      you are strongly encouraged not to change.
    
 
- null
- Enable only the "NULL" OpenSSL ciphers, these provide
      authentication without encryption. This setting is only appropriate in the
      rare case that all clients are prepared to use NULL ciphers (not normally
      enabled in TLS clients). The underlying cipherlist is specified via the
      tls_null_cipherlist configuration parameter, which you are strongly
      encouraged not to change.
    
 
- low
- Enable "LOW" grade or stronger OpenSSL ciphers. In Postfix >=
      3.8 this cipher grade is always identical to "medium". Recent
      versions of OpenSSL do not support any "LOW" grade ciphers. In
      earlier Postfix releases the underlying cipherlist was specified via the
      tls_low_cipherlist configuration parameter, which you are strongly
      encouraged not to change. This obsolete cipher grade SHOULD NOT be used.
    
 
- export
- Enable "EXPORT" grade or stronger OpenSSL ciphers. In Postfix
      >= 3.8 this cipher grade is always identical to "medium".
      Recent versions of OpenSSL do not support any "EXPORT" grade
      ciphers. In earlier Postfix releases the underlying cipherlist was
      specified via the tls_export_cipherlist configuration parameter, which you
      are strongly encouraged not to change. This obsolete cipher grade SHOULD
      NOT be used.
    
 
Cipher types listed in smtpd_tls_mandatory_exclude_ciphers or
    smtpd_tls_exclude_ciphers are excluded from the base definition of the
    selected cipher grade. See smtpd_tls_ciphers for cipher controls that apply
    to opportunistic TLS.
The underlying cipherlists for grades other than "null"
    include anonymous ciphers, but these are automatically filtered out if the
    server is configured to ask for remote SMTP client certificates. You are
    very unlikely to need to take any steps to exclude anonymous ciphers, they
    are excluded automatically as required. If you must exclude anonymous
    ciphers even when Postfix does not need or use peer certificates, set
    "smtpd_tls_exclude_ciphers = aNULL". To exclude anonymous ciphers
    only when TLS is enforced, set "smtpd_tls_mandatory_exclude_ciphers =
    aNULL".
This feature is available in Postfix 2.3 and later.
Additional list of ciphers or cipher types to exclude from the Postfix SMTP
  server cipher list at mandatory TLS security levels. This list works in
  addition to the exclusions listed with smtpd_tls_exclude_ciphers (see there
  for syntax details).
This feature is available in Postfix 2.3 and later.
TLS protocols accepted by the Postfix SMTP server with mandatory TLS encryption.
  If the list is empty, the server supports all available TLS protocol versions.
  A non-empty value is a list of protocol names to include or exclude, separated
  by whitespace, commas or colons.
The valid protocol names (see SSL_get_version(3)) are
    "SSLv2", "SSLv3", "TLSv1",
    "TLSv1.1", "TLSv1.2" and "TLSv1.3". Starting
    with Postfix 3.6, the default value is ">=TLSv1", which sets
    TLS 1.0 as the lowest supported TLS protocol version (see below). Older
    releases use the "!" exclusion syntax, also described below.
As of Postfix 3.6, the preferred way to limit the range of
    acceptable protocols is to set the lowest acceptable TLS protocol version
    and/or the highest acceptable TLS protocol version. To set the lower bound
    include an element of the form: ">=version" where
    version is a either one of the TLS protocol names listed above, or a
    hexadecimal number corresponding to the desired TLS protocol version (0301
    for TLS 1.0, 0302 for TLS 1.1, etc.). For the upper bound, use
    "<=version". There must be no whitespace between the
    ">=" or "<=" symbols and the protocol name or
    number.
Hexadecimal protocol numbers make it possible to specify protocol
    bounds for TLS versions that are known to OpenSSL, but might not be known to
    Postfix. They cannot be used with the legacy exclusion syntax. Leading
    "0" or "0x" prefixes are supported, but not required.
    Therefore, "301", "0301", "0x301" and
    "0x0301" are all equivalent to "TLSv1". Hexadecimal
    versions unknown to OpenSSL will fail to set the upper or lower bound, and a
    warning will be logged. Hexadecimal versions should only be used when
    Postfix is linked with some future version of OpenSSL that supports TLS 1.4
    or later, but Postfix does not yet support a symbolic name for that protocol
    version.
Hexadecimal example (Postfix >= 3.6):
  
# Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtpd_tls_mandatory_protocols = >=TLSv1.2, <=0305
# Allow only TLS 1.2 and up:
smtpd_tls_mandatory_protocols = >=0x0303
With Postfix < 3.6 there is no support for a minimum or maximum
    version, and the protocol range is configured via protocol exclusions. To
    require at least TLS 1.0, set "smtpd_tls_mandatory_protocols = !SSLv2,
    !SSLv3". Listing the protocols to include, rather than protocols to
    exclude, is supported, but not recommended. The exclusion form more
    accurately matches the underlying OpenSSL interface.
Support for "TLSv1.3" was introduced in OpenSSL 1.1.1.
    Disabling this protocol via "!TLSv1.3" is supported since Postfix
    3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).
Example:
# Preferred syntax with Postfix >= 3.6:
smtpd_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
# Legacy syntax:
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
This feature is available in Postfix 2.3 and later.
TLS protocols accepted by the Postfix SMTP server with opportunistic TLS
  encryption. If the list is empty, the server supports all available TLS
  protocol versions. A non-empty value is a list of protocol names to include or
  exclude, separated by whitespace, commas or colons.
The valid protocol names (see SSL_get_version(3)) are
    "SSLv2", "SSLv3", "TLSv1",
    "TLSv1.1", "TLSv1.2" and "TLSv1.3". Starting
    with Postfix 3.6, the default value is ">=TLSv1", which sets
    TLS 1.0 as the lowest supported TLS protocol version (see below). Older
    releases use the "!" exclusion syntax, also described below.
As of Postfix 3.6, the preferred way to limit the range of
    acceptable protocols is to set the lowest acceptable TLS protocol version
    and/or the highest acceptable TLS protocol version. To set the lower bound
    include an element of the form: ">=version" where
    version is a either one of the TLS protocol names listed above, or a
    hexadecimal number corresponding to the desired TLS protocol version (0301
    for TLS 1.0, 0302 for TLS 1.1, etc.). For the upper bound, use
    "<=version". There must be no whitespace between the
    ">=" or "<=" symbols and the protocol name or
    number.
Hexadecimal protocol numbers make it possible to specify protocol
    bounds for TLS versions that are known to OpenSSL, but might not be known to
    Postfix. They cannot be used with the legacy exclusion syntax. Leading
    "0" or "0x" prefixes are supported, but not required.
    Therefore, "301", "0301", "0x301" and
    "0x0301" are all equivalent to "TLSv1". Hexadecimal
    versions unknown to OpenSSL will fail to set the upper or lower bound, and a
    warning will be logged. Hexadecimal versions should only be used when
    Postfix is linked with some future version of OpenSSL that supports TLS 1.4
    or later, but Postfix does not yet support a symbolic name for that protocol
    version.
Hexadecimal example (Postfix >= 3.6):
  
# Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
# in some future version of OpenSSL (presently a warning is logged).
smtpd_tls_protocols = >=TLSv1, <=0305
# Allow only TLS 1.0 and up:
smtpd_tls_protocols = >=0x0301
With Postfix < 3.6 there is no support for a minimum or maximum
    version, and the protocol range is configured via protocol exclusions. To
    require at least TLS 1.0, set "smtpd_tls_protocols = !SSLv2,
    !SSLv3". Listing the protocols to include, rather than protocols to
    exclude, is supported, but not recommended. The exclusion form more
    accurately matches the underlying OpenSSL interface.
Support for "TLSv1.3" was introduced in OpenSSL 1.1.1.
    Disabling this protocol via "!TLSv1.3" is supported since Postfix
    3.4 (or patch releases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).
Example:
# Preferred syntax with Postfix >= 3.6:
smtpd_tls_protocols = >=TLSv1, <=TLSv1.3
# Legacy syntax:
smtpd_tls_protocols = !SSLv2, !SSLv3
This feature is available in Postfix 2.6 and later.
Request that the Postfix SMTP server produces Received: message headers that
  include information about the protocol and cipher used, as well as the remote
  SMTP client CommonName and client certificate issuer CommonName. This is
  disabled by default, as the information may be modified in transit through
  other mail servers. Only information that was recorded by the final
  destination can be trusted.
This feature is available in Postfix 2.2 and later.
With mandatory TLS encryption, require a trusted remote SMTP client certificate
  in order to allow TLS connections to proceed. This option implies
  "smtpd_tls_ask_ccert = yes".
When TLS encryption is optional, this setting is ignored with a
    warning written to the mail log.
This feature is available in Postfix 2.2 and later.
The SMTP TLS security level for the Postfix SMTP server; when a non-empty value
  is specified, this overrides the obsolete parameters smtpd_use_tls and
  smtpd_enforce_tls. This parameter is ignored with "smtpd_tls_wrappermode
  = yes".
Specify one of the following security levels:
  - none
- TLS will not be used.
    
 
- may
- Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but
      do not require that clients use TLS encryption.
    
 
- encrypt
- Mandatory TLS encryption: announce STARTTLS support to remote SMTP
      clients, and require that clients use TLS encryption. According to RFC
      2487 this MUST NOT be applied in case of a publicly-referenced SMTP
      server. Instead, this option should be used only on dedicated servers.
    
 
Note 1: the "fingerprint", "verify" and
    "secure" levels are not supported here. The Postfix SMTP server
    logs a warning and uses "encrypt" instead. To verify remote SMTP
    client certificates, see TLS_README for a discussion of the
    smtpd_tls_ask_ccert, smtpd_tls_req_ccert, and permit_tls_clientcerts
    features.
Note 2: The parameter setting "smtpd_tls_security_level =
    encrypt" implies "smtpd_tls_auth_only = yes".
Note 3: when invoked via "sendmail -bs", Postfix will
    never offer STARTTLS due to insufficient privileges to access the server
    private key. This is intended behavior.
This feature is available in Postfix 2.3 and later.
Name of the file containing the optional Postfix SMTP server TLS session cache.
  Specify a database type that supports enumeration, such as btree or
  sdbm; there is no need to support concurrent access. The file is
  created if it does not exist. The smtpd(8) daemon does not use this
  parameter directly, rather the cache is implemented indirectly in the
  tlsmgr(8) daemon. This means that per-smtpd-instance master.cf
  overrides of this parameter are not effective. Note that each of the cache
  databases supported by tlsmgr(8) daemon:
  $smtpd_tls_session_cache_database, $smtp_tls_session_cache_database (and with
  Postfix 2.3 and later $lmtp_tls_session_cache_database), needs to be stored
  separately. It is not at this time possible to store multiple caches in a
  single database.
Note: dbm databases are not suitable. TLS session objects
    are too large.
As of version 2.5, Postfix no longer uses root privileges when
    opening this file. The file should now be stored under the Postfix-owned
    data_directory. As a migration aid, an attempt to open the file under a
    non-Postfix directory is redirected to the Postfix-owned data_directory, and
    a warning is logged.
As of Postfix 2.11 the preferred mechanism for session resumption
    is RFC 5077 TLS session tickets, which don't require server-side storage.
    Consequently, for Postfix >= 2.11 this parameter should generally be left
    empty. TLS session tickets require an OpenSSL library (at least version
    0.9.8h) that provides full support for this TLS extension. See also
    smtpd_tls_session_cache_timeout.
Example:
smtpd_tls_session_cache_database = btree:/var/db/postfix/smtpd_scache
This feature is available in Postfix 2.2 and later.
The expiration time of Postfix SMTP server TLS session cache information. A
  cache cleanup is performed periodically every $smtpd_tls_session_cache_timeout
  seconds. As with $smtpd_tls_session_cache_database, this parameter is
  implemented in the tlsmgr(8) daemon and therefore per-smtpd-instance
  master.cf overrides are not possible.
As of Postfix 2.11 this setting cannot exceed 100 days. If set
    <= 0, session caching is disabled, not just via the database, but also
    via RFC 5077 TLS session tickets, which don't require server-side storage.
    If set to a positive value less than 2 minutes, the minimum value of 2
    minutes is used instead. TLS session tickets require an OpenSSL library (at
    least version 0.9.8h) that provides full support for this TLS extension.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
This feature is available in Postfix 2.2 and later, and updated
    for TLS session ticket support in Postfix 2.11.
Run the Postfix SMTP server in TLS "wrapper" mode, instead of using
  the STARTTLS command.
If you want to support this service, enable a special port in
    master.cf, and specify "-o smtpd_tls_wrappermode=yes" on the SMTP
    server's command line. Port 465 (submissions/smtps) is reserved for this
    purpose.
This feature is available in Postfix 2.2 and later.
The name of the proxy protocol used by an optional before-smtpd proxy agent.
  When a proxy agent is used, this protocol conveys local and remote address and
  port information. Specify "smtpd_upstream_proxy_protocol = haproxy"
  to enable the haproxy protocol; version 2 is supported with Postfix 3.5 and
  later.
NOTE: To use the nginx proxy with smtpd(8), enable the
    XCLIENT protocol with smtpd_authorized_xclient_hosts. This supports SASL
    authentication in the proxy agent (Postfix 2.9 and later).
This feature is available in Postfix 2.10 and later.
The time limit for the proxy protocol specified with the
  smtpd_upstream_proxy_protocol parameter.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.10 and later.
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not
  require that clients use TLS encryption.
Note: when invoked via "sendmail -bs", Postfix
    will never offer STARTTLS due to insufficient privileges to access the
    server private key. This is intended behavior.
This feature is available in Postfix 2.2 and later. With Postfix
    2.3 and later use smtpd_tls_security_level instead.
Detect that a message requires SMTPUTF8 support for the specified mail origin
  classes. This is a workaround to avoid chicken-and-egg problems during the
  initial SMTPUTF8 roll-out in environments with pre-existing mail flows that
  contain UTF8. Those mail flows should not break because Postfix suddenly
  refuses to deliver such mail to down-stream MTAs that don't announce SMTPUTF8
  support.
The problem is that Postfix cannot rely solely on the sender's
    declaration that a message requires SMTPUTF8 support, because UTF8 may be
    introduced during local processing (for example, the client hostname in
    Postfix's Received: header, adding @$myorigin or .$mydomain to an incomplete
    address, address rewriting, alias expansion, automatic BCC recipients, local
    forwarding, and changes made by header checks or Milter applications).
For now, the default is to enable "SMTPUTF8 required"
    autodetection only for Postfix sendmail command-line submissions and address
    verification probes. This may change once SMTPUTF8 support achieves world
    domination. However, sites that add UTF8 content via local processing (see
    above) should autodetect the need for SMTPUTF8 support for all email.
Specify one or more of the following:
  -  sendmail 
- Submission with the Postfix sendmail(1) command.
    
 
-  smtpd 
- Mail received with the smtpd(8) daemon.
    
 
-  qmqpd 
- Mail received with the qmqpd(8) daemon.
    
 
-  forward 
- Local forwarding or aliasing. When a message is received with
      "SMTPUTF8 required", then the forwarded (aliased) message always
      has "SMTPUTF8 required".
    
 
-  bounce 
- Submission by the bounce(8) daemon. When a message is received with
      "SMTPUTF8 required", then the delivery status notification
      always has "SMTPUTF8 required".
    
 
-  notify 
- Postmaster notification from the smtp(8) or smtpd(8) daemon.
    
 
-  verify 
- Address verification probe from the verify(8) daemon.
    
 
-  all 
- Enable SMTPUTF8 autodetection for all mail.
    
 
This feature is available in Postfix 3.0 and later.
Enable preliminary SMTPUTF8 support for the protocols described in RFC 6531, RFC
  6532, and RFC 6533. This requires that Postfix is built to support these
  protocols.
This feature is available in Postfix 3.0 and later.
Safety net to keep mail queued that would otherwise be returned to the sender.
  This parameter disables locally-generated bounces, changes the handling of
  negative responses from remote servers, content filters or plugins, and
  prevents the Postfix SMTP server from rejecting mail permanently by changing
  5xx reply codes into 4xx. However, soft_bounce is no cure for address
  rewriting mistakes or mail routing mistakes.
Note: "soft_bounce = yes" is in some cases implemented
    by modifying server responses. Therefore, the response that Postfix logs may
    differ from the response that Postfix actually sends or receives.
Example:
soft_bounce = yes
The time after which a stale exclusive mailbox lockfile is removed. This is used
  for delivery to file or mailbox.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is documented in the STRESS_README document.
This feature is available in Postfix 2.5 and later.
Reject mail with 8-bit text in message headers. This blocks mail from poorly
  written applications.
This feature should not be enabled on a general purpose mail
    server, because it is likely to reject legitimate email.
This feature is available in Postfix 2.0 and later.
Enable both strict_7bit_headers and strict_8bitmime_body.
This feature should not be enabled on a general purpose mail
    server, because it is likely to reject legitimate email.
This feature is available in Postfix 2.0 and later.
Reject 8-bit message body text without 8-bit MIME content encoding information.
  This blocks mail from poorly written applications.
Unfortunately, this also rejects majordomo approval requests when
    the included request contains valid 8-bit MIME mail, and it rejects bounces
    from mailers that do not MIME encapsulate 8-bit content (for example,
    bounces from qmail or from old versions of Postfix).
This feature should not be enabled on a general purpose mail
    server, because it is likely to reject legitimate email.
This feature is available in Postfix 2.0 and later.
Defer delivery when a mailbox file is not owned by its recipient. The default
  setting is not backwards compatible.
This feature is available in Postfix 2.5.3 and later.
Reject mail with invalid Content-Transfer-Encoding: information for the
  message/* or multipart/* MIME content types. This blocks mail from poorly
  written software.
This feature should not be enabled on a general purpose mail
    server, because it will reject mail after a single violation.
This feature is available in Postfix 2.0 and later.
Require that addresses received in SMTP MAIL FROM and RCPT TO commands are
  enclosed with <>, and that those addresses do not contain RFC 822 style
  comments or phrases. This stops mail from poorly written software.
By default, the Postfix SMTP server accepts RFC 822 syntax in MAIL
    FROM and RCPT TO addresses.
Enable stricter enforcement of the SMTPUTF8 protocol. The Postfix SMTP server
  accepts UTF8 sender or recipient addresses only when the client requests an
  SMTPUTF8 mail transaction.
This feature is available in Postfix 3.0 and later.
Obsolete SUN mailtool compatibility feature. Instead, use
  "mailbox_delivery_lock = dotlock".
Enable the rewriting of "site!user" into "user@site". This
  is necessary if your machine is connected to UUCP networks. It is enabled by
  default.
Note: with Postfix version 2.2, message header address rewriting
    happens only when one of the following conditions is true:
  - The message is received with the Postfix sendmail(1) command,
- The message is received from a network client that matches
      $local_header_rewrite_clients,
- The message is received from the network, and the
      remote_header_rewrite_domain parameter specifies a non-empty value.
    
 
To get the behavior before Postfix version 2.2, specify
    "local_header_rewrite_clients = static:all".
Example:
swap_bangpath = no
The syslog facility of Postfix logging. Specify a facility as defined in
  syslog.conf(5). The default facility is "mail".
Warning: a non-default syslog_facility setting takes effect only
    after a Postfix process has completed initialization. Errors during process
    initialization will be logged with the default facility. Examples are errors
    while parsing the command line arguments, and errors while accessing the
    Postfix main.cf configuration file.
A prefix that is prepended to the process name in syslog records, so that, for
  example, "smtpd" becomes "prefix/smtpd".
Warning: a non-default syslog_name setting takes effect only after
    a Postfix process has completed initialization. Errors during process
    initialization will be logged with the default name. Examples are errors
    while parsing the command line arguments, and errors while accessing the
    Postfix main.cf configuration file.
An optional workaround for routers that break TCP window scaling. Specify a
  value > 0 and < 65536 to enable this feature. With Postfix TCP servers
  (smtpd(8), qmqpd(8)), this feature is implemented by the Postfix
  master(8) daemon.
To change this parameter without stopping Postfix, you need to
    first terminate all Postfix TCP servers:
  
# postconf -e master_service_disable=inet
# postfix reload
This immediately terminates all processes that accept network
    connections. Next, you enable Postfix TCP servers with the updated
    tcp_windowsize setting:
  
# postconf -e tcp_windowsize=65535 master_service_disable=
# postfix reload
If you skip these steps with a running Postfix system, then the
    tcp_windowsize change will work only for Postfix TCP clients
    (smtp(8), lmtp(8)).
This feature is available in Postfix 2.6 and later.
Append the system-supplied default Certification Authority certificates to the
  ones specified with *_tls_CApath or *_tls_CAfile. The default is
  "no"; this prevents Postfix from trusting third-party certificates
  and giving them relay permission with permit_tls_all_clientcerts.
This feature is available in Postfix 2.4.15, 2.5.11, 2.6.8, 2.7.2
    and later versions. Specify "tls_append_default_CA = yes" for
    backwards compatibility, to avoid breaking certificate verification with
    sites that don't use permit_tls_all_clientcerts.
Optional configuration file with baseline OpenSSL settings. OpenSSL loads any
  SSL settings found in the configuration file for the selected application name
  (see tls_config_name) or else the built-in application name
  "openssl_conf" when no application name is specified, or no
  corresponding configuration section is present.
With OpenSSL releases 1.1.1 and 1.1.1a, applications (including
    Postfix) can neither specify an alternative configuration file, nor avoid
    loading the default configuration file.
With OpenSSL 1.1.1b or later, this parameter may be set to one
  of:
  - default (default)
- Load the system-wide "openssl.cnf" configuration file.
    
 
- none (recommended, OpenSSL 1.1.1b or later only)
- This setting disables loading of the system-wide "openssl.cnf"
      file.
    
 
- /absolute-path (OpenSSL 1.1.1b or later only)
- Load the configuration file specified by /absolute-path. With this
      setting it is an error for the file to not contain any settings for the
      selected tls_config_name. There is no fallback to the default
      "openssl_conf" name.
    
 
Failures in processing of the built-in default configuration file,
    are silently ignored. Any errors in loading a non-default configuration file
    are detected by Postfix, and cause TLS support to be disabled.
The OpenSSL configuration file format is not documented here,
    beyond giving two examples.
Example: Default settings for all applications.
  
# The name 'openssl_conf' is the default application name
# The section name to the right of the '=' sign is arbitrary,
# any name will do, so long as it refers to the desired section.
#
# The name 'system_default' selects the settings applied internally
# by the SSL library as part of SSL object creation.  Applications
# can then apply any additional settings of their choice.
#
# In this example, TLS versions prior to 1.2 are disabled by default.
#
openssl_conf = system_wide_settings
[system_wide_settings]
ssl_conf = ssl_library_settings
[ssl_library_settings]
system_default = initial_ssl_settings
[initial_ssl_settings]
MinProtocol = TLSv1.2
Example: Custom settings for an application named
    "postfix".
  
# The mapping from an application name to the corresponding configuration
# section must appear near the top of the file, (in what is sometimes called
# the "default section") prior to the start of any explicitly named
# "[sections]".  The named sections can appear in any order and don't nest.
#
postfix = postfix_settings
[postfix_settings]
ssl_conf = postfix_ssl_settings
[postfix_ssl_settings]
system_default = baseline_postfix_settings
[baseline_postfix_settings]
MinProtocol = TLSv1
This feature is available in Postfix >= 3.9, 3.8.1, 3.7.6,
    3.6.10, and 3.5.20.
The application name passed by Postfix to OpenSSL library initialization
  functions. This name is used to select the desired configuration
  "section" in the OpenSSL configuration file specified via the
  tls_config_file parameter. When empty, or when the selected name is not
  present in the configuration file, the default application name
  ("openssl_conf") is used as a fallback.
This feature is available in Postfix >= 3.9, 3.8.1, 3.7.6,
    3.6.10, and 3.5.20.
The number of pseudo-random bytes that an smtp(8) or smtpd(8)
  process requests from the tlsmgr(8) server in order to seed its
  internal pseudo random number generator (PRNG). The default of 32 bytes
  (equivalent to 256 bits) is sufficient to generate a 128bit (or 168bit)
  session key.
This feature is available in Postfix 2.2 and later.
Configure RFC7671 DANE TLSA digest algorithm agility. Do not change this setting
  from its default value.
See Section 8 of RFC7671 for correct key rotation procedures.
This feature is available in Postfix 2.11 through 3.1. Postfix 3.2
    and later ignore this configuration parameter and behave as though it were
    set to "on".
DANE TLSA (RFC 6698, RFC 7671, RFC 7672) resource-record "matching
  type" digest algorithms in descending preference order. All the specified
  algorithms must be supported by the underlying OpenSSL library, otherwise the
  Postfix SMTP client will not support DANE TLSA security.
Specify a list of digest names separated by commas and/or
    whitespace. Each digest name may be followed by an optional
    "=<number>" suffix. For example, "sha512" may
    instead be specified as "sha512=2" and "sha256" may
    instead be specified as "sha256=1". The optional number must match
    the <a
    href="https://www.iana.org/assignments/dane-parameters/dane-parameters.xhtml#matching-types"
    >IANA assigned TLSA matching type number the algorithm in question.
    Postfix will check this constraint for the algorithms it knows about.
    Additional matching type algorithms registered with IANA can be added with
    explicit numbers provided they are supported by OpenSSL.
Invalid list elements are logged with a warning and disable DANE
    support. TLSA RRs that specify digests not included in the list are ignored
    with a warning.
Note: It is unwise to omit sha256 from the digest list. This
    digest algorithm is the only mandatory to implement digest algorithm in RFC
    6698, and many servers are expected to publish TLSA records with just sha256
    digests. Unless one of the standard digests is seriously compromised and
    servers have had ample time to update their TLSA records you should not omit
    any standard digests, just arrange them in order from strongest to
  weakest.
This feature is available in Postfix 2.11 and later.
Enable support for RFC 6698 (DANE TLSA) DNS records that contain digests of
  trust-anchors with certificate usage "2". Do not change this setting
  from its default value.
This feature is available in Postfix 2.11 through 3.1. It has been
    withdrawn in Postfix 3.2, as trust-anchor TLSA records are now widely used
    and have proved sufficiently reliable. Postfix 3.2 and later ignore this
    configuration parameter and behaves as though it were set to
    "yes".
List or bit-mask of OpenSSL bug work-arounds to disable.
The OpenSSL toolkit includes a set of work-arounds for buggy
    SSL/TLS implementations. Applications, such as Postfix, that want to
    maximize interoperability ask the OpenSSL library to enable the full set of
    recommended work-arounds.
From time to time, it is discovered that a work-around creates a
    security issue, and should no longer be used. If upgrading OpenSSL to a
    fixed version is not an option or an upgrade is not available in a timely
    manner, or in closed environments where no buggy clients or servers exist,
    it may be appropriate to disable some or all of the OpenSSL interoperability
    work-arounds. This parameter specifies which bug work-arounds to
  disable.
If the value of the parameter is a hexadecimal long integer
    starting with "0x", the bug work-arounds corresponding to the bits
    specified in its value are removed from the SSL_OP_ALL work-around
    bit-mask (see openssl/ssl.h and SSL_CTX_set_options(3)). You can
    specify more bits than are present in SSL_OP_ALL, excess bits are ignored.
    Specifying 0xFFFFFFFF disables all bug-workarounds on a 32-bit system. This
    should also be sufficient on 64-bit systems, until OpenSSL abandons support
    for 32-bit systems and starts using the high 32 bits of a 64-bit
    bug-workaround mask.
Otherwise, the parameter is a white-space or comma separated list
    of specific named bug work-arounds chosen from the list below. It is
    possible that your OpenSSL version includes new bug work-arounds added after
    your Postfix source code was last updated, in that case you can only disable
    one of these via the hexadecimal syntax above.
  - CRYPTOPRO_TLSEXT_BUG
- New with GOST support in OpenSSL 1.0.0.
    
 
- DONT_INSERT_EMPTY_FRAGMENTS
- See SSL_CTX_set_options(3)
    
 
- LEGACY_SERVER_CONNECT
- See SSL_CTX_set_options(3)
    
 
- MICROSOFT_BIG_SSLV3_BUFFER
- See SSL_CTX_set_options(3)
    
 
- MICROSOFT_SESS_ID_BUG
- See SSL_CTX_set_options(3)
    
 
- MSIE_SSLV2_RSA_PADDING
- also aliased as CVE-2005-2969. Postfix 2.8 disables this
      work-around by default with OpenSSL versions that may predate the fix.
      Fixed in OpenSSL 0.9.7h and OpenSSL 0.9.8a.
    
 
- NETSCAPE_CHALLENGE_BUG
- See SSL_CTX_set_options(3)
    
 
- NETSCAPE_REUSE_CIPHER_CHANGE_BUG
- also aliased as CVE-2010-4180. Postfix 2.8 disables this
      work-around by default with OpenSSL versions that may predate the fix.
      Fixed in OpenSSL 0.9.8q and OpenSSL 1.0.0c.
    
 
- SSLEAY_080_CLIENT_DH_BUG
- See SSL_CTX_set_options(3)
    
 
- SSLREF2_REUSE_CERT_TYPE_BUG
- See SSL_CTX_set_options(3)
    
 
- TLS_BLOCK_PADDING_BUG
- See SSL_CTX_set_options(3)
    
 
- TLS_D5_BUG
- See SSL_CTX_set_options(3)
    
 
- TLS_ROLLBACK_BUG
- See SSL_CTX_set_options(3). This is disabled in OpenSSL 0.9.7 and
      later. Nobody should still be using 0.9.6!
    
 
- TLSEXT_PADDING
- Postfix >= 3.4. See SSL_CTX_set_options(3).
    
 
This feature is available in Postfix 2.8 and later.
The prioritized list of elliptic curves supported by the Postfix SMTP client and
  server. These curves are used by the Postfix SMTP server when
  "smtpd_tls_eecdh_grade = auto". The selected curves must be
  implemented by OpenSSL and be standardized for use in TLS (RFC 8422). It is
  unwise to list only "bleeding-edge" curves supported by a small
  subset of clients. The default list is suitable for most users.
Postfix skips curve names that are unknown to OpenSSL, or that are
    known but not yet implemented. This makes it possible to
    "anticipate" support for curves that should be used once they
    become available. In particular, in some OpenSSL versions, the new RFC 8031
    curves "X25519" and "X448" may be known by name, but
    ECDH support for either or both may be missing. These curves may appear in
    the default value of this parameter, even though they'll only be usable with
    later versions of OpenSSL.
See also the "tls_ffdhe_auto_groups" parameter, which
    supports customizing the list of FFDHE groups enabled with TLS 1.3. That
    setting is introduced with Postfix 3.8, when built against OpenSSL 3.0 or
    later.
This feature is available in Postfix 3.2 and later, when it is
    compiled and linked with OpenSSL 1.0.2 or later on platforms where EC
    algorithms have not been disabled by the vendor.
The elliptic curve used by the Postfix SMTP server for sensibly strong ephemeral
  ECDH key exchange. This curve is used by the Postfix SMTP server when
  "smtpd_tls_eecdh_grade = strong". The phrase "sensibly
  strong" means approximately 128-bit security based on best known attacks.
  The selected curve must be implemented by OpenSSL (as reported by
  ecparam(1) with the "-list_curves" option) and be one of the
  curves listed in Section 5.1.1 of RFC 8422. You should not generally change
  this setting. Remote SMTP client implementations must support this curve for
  EECDH key exchange to take place. It is unwise to choose only
  "bleeding-edge" curves supported by only a small subset of clients.
The default "strong" curve is rated in NSA Suite B for
    information classified up to SECRET.
Note: elliptic curve names are poorly standardized; different
    standards groups are assigning different names to the same underlying
    curves. The curve with the X9.62 name "prime256v1" is also known
    under the SECG name "secp256r1", but OpenSSL does not recognize
    the latter name.
If you want to take maximal advantage of ciphers that offer
    forward secrecy see the Getting started section of FORWARD_SECRECY_README.
    The full document conveniently presents all information about Postfix
    "perfect" forward secrecy support in one place: what forward
    secrecy is, how to tweak settings, and what you can expect to see when
    Postfix uses ciphers with forward secrecy.
This feature is available in Postfix 2.6 and later, when it is
    compiled and linked with OpenSSL 1.0.0 or later on platforms where EC
    algorithms have not been disabled by the vendor.
The elliptic curve used by the Postfix SMTP server for maximally strong
  ephemeral ECDH key exchange. This curve is used by the Postfix SMTP server
  when "smtpd_tls_eecdh_grade = ultra". The phrase "maximally
  strong" means approximately 192-bit security based on best known attacks.
  This additional strength comes at a significant computational cost, most users
  should instead set "smtpd_tls_eecdh_grade = strong". The selected
  curve must be implemented by OpenSSL (as reported by ecparam(1) with
  the "-list_curves" option) and be one of the curves listed in
  Section 5.1.1 of RFC 8422. You should not generally change this setting.
  Remote SMTP client implementations must support this curve for EECDH key
  exchange to take place. It is unwise to choose only "bleeding-edge"
  curves supported by only a small subset of clients.
This default "ultra" curve is rated in NSA Suite B for
    information classified up to TOP SECRET.
If you want to take maximal advantage of ciphers that offer
    forward secrecy see the Getting started section of FORWARD_SECRECY_README.
    The full document conveniently presents all information about Postfix
    "perfect" forward secrecy support in one place: what forward
    secrecy is, how to tweak settings, and what you can expect to see when
    Postfix uses ciphers with forward secrecy.
This feature is available in Postfix 2.6 and later, when it is
    compiled and linked with OpenSSL 1.0.0 or later on platforms where EC
    algorithms have not been disabled by the vendor.
The OpenSSL cipherlist for "export" or higher grade ciphers. Ignored
  as of Postfix 3.8. In earlier Postfix releases this defined the meaning of the
  "export" setting in smtpd_tls_ciphers, smtpd_tls_mandatory_ciphers,
  smtp_tls_ciphers, smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and
  lmtp_tls_mandatory_ciphers. You are strongly encouraged not to change this
  setting.
This feature is available in Postfix 2.3 and later.
A workaround for implementations that hang Postfix while shutting down a TLS
  session, until Postfix times out. With this enabled, Postfix will not wait for
  the remote TLS peer to respond to a TLS 'close' notification. This behavior is
  recommended for TLSv1.0 and later.
The prioritized list of finite-field Diffie-Hellman ephemeral (FFDHE) key
  exchange groups supported by the Postfix SMTP client and server. OpenSSL 3.0
  adds support for FFDHE key agreement in TLS 1.3. In OpenSSL 1.1.1, TLS 1.3 was
  only supported with elliptic-curve based key agreement. The
  "tls_ffdhe_auto_groups" parameter makes it possible to configure the
  list of FFDHE groups that the Postfix client or server will enable in OpenSSL
  3.0 and up. This parameter has no effect when Postfix is built against earlier
  OpenSSL versions.
The default list of FFDHE groups that Postfix enables in OpenSSL
    3.0 and up includes just the 2048 and 3072-bit groups. Stronger FFDHE groups
    perform poorly and EC groups are a much better choice for the same security
    level. Postfix ignores group names that are unknown to OpenSSL, or that are
    known but not yet implemented. The FFDHE groups are largely a backup, in
    case some peer does not support EC key exchange, or EC key exchange needs to
    be disabled for some pressing reason.
Setting this parameter empty disables FFDHE support in TLS 1.3.
    Whether FFDHE key agreement is enabled in TLS 1.2 and earlier depends on
    whether any of the "kDHE" ciphers are included in the
  cipherlist.
Conversely, setting "tls_eecdh_auto_curves" empty
    disables TLS 1.3 EC key agreement in OpenSSL 3.0 and later. Note that at
    least one of "tls_eecdh_auto_curves" and
    "tls_ffdhe_auto_groups" must be non-empty, this is required by
    OpenSSL 3.0. If both are inadvertently set empty, Postfix will fall back to
    the compiled-in defaults.
All the default groups and EC curves should sufficiently strong to
    make "pruning" the defaults unwise. At a minimum,
    "X25519" and "P-256" (a.k.a. "prime256v1")
    should be among the enabled EC curves, while "dhe2048" and
    "dhe3072" should be among the FFDHE groups.
This feature is available in Postfix 3.8 and later, when it is
    compiled and linked with OpenSSL 3.0 or later.
The OpenSSL cipherlist for "high" grade ciphers. This defines the
  meaning of the "high" setting in smtpd_tls_ciphers,
  smtpd_tls_mandatory_ciphers, smtp_tls_ciphers, smtp_tls_mandatory_ciphers,
  lmtp_tls_ciphers, and lmtp_tls_mandatory_ciphers. You are strongly encouraged
  not to change this setting.
This feature is available in Postfix 2.3 and later.
A temporary migration aid for sites that use certificate public-key
  fingerprints with Postfix 2.9.0..2.9.5, which use an incorrect algorithm. This
  parameter has no effect on the certificate fingerprint support that is
  available since Postfix 2.2.
Specify "tls_legacy_public_key_fingerprints = yes"
    temporarily, pending a migration from configuration files with incorrect
    Postfix 2.9.0..2.9.5 certificate public-key finger prints, to the correct
    fingerprints used by Postfix 2.9.6 and later. To compute the correct
    certificate public-key fingerprints, see TLS_README.
This feature is available in Postfix 2.9.6 and later.
The OpenSSL cipherlist for "low" or higher grade ciphers. Ignored as
  of Postfix 3.8. In earlier Postfix releases this defined the meaning of the
  "low" setting in smtpd_tls_ciphers, smtpd_tls_mandatory_ciphers,
  smtp_tls_ciphers, smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and
  lmtp_tls_mandatory_ciphers. You are strongly encouraged not to change this
  setting.
This feature is available in Postfix 2.3 and later.
The OpenSSL cipherlist for "medium" or higher grade ciphers. This
  defines the meaning of the "medium" setting in smtpd_tls_ciphers,
  smtpd_tls_mandatory_ciphers, smtp_tls_ciphers, smtp_tls_mandatory_ciphers,
  lmtp_tls_ciphers, and lmtp_tls_mandatory_ciphers. This is the default
  cipherlist for mandatory TLS encryption in the TLS client (with anonymous
  ciphers disabled when verifying server certificates). This is the default
  cipherlist for opportunistic TLS with Postfix releases after the middle of
  2015. You are strongly encouraged not to change this setting.
This feature is available in Postfix 2.3 and later.
The OpenSSL cipherlist for "NULL" grade ciphers that provide
  authentication without encryption. This defines the meaning of the
  "null" setting in smtpd_tls_mandatory_ciphers,
  smtp_tls_mandatory_ciphers and lmtp_tls_mandatory_ciphers. You are strongly
  encouraged not to change this setting.
This feature is available in Postfix 2.3 and later.
With SSLv3 and later, use the Postfix SMTP server's cipher preference order
  instead of the remote client's cipher preference order.
By default, the OpenSSL server selects the client's most preferred
    cipher that the server supports. With SSLv3 and later, the server may choose
    its own most preferred cipher that is supported (offered) by the client.
    Setting "tls_preempt_cipherlist = yes" enables server cipher
    preferences.
While server cipher selection may in some cases lead to a more
    secure or performant cipher choice, there is some risk of interoperability
    issues. In the past, some SSL clients have listed lower priority ciphers
    that they did not implement correctly. If the server chooses a cipher that
    the client prefers less, it may select a cipher whose client implementation
    is flawed. Most notably Windows 2003 Microsoft Exchange servers have flawed
    implementations of DES-CBC3-SHA, which OpenSSL considers stronger than
    RC4-SHA. Enabling server cipher-suite selection may create interoperability
    issues with Windows 2003 Microsoft Exchange clients.
This feature is available in Postfix 2.8 and later, in combination
    with OpenSSL 0.9.7 and later.
The number of bytes that tlsmgr(8) reads from $tls_random_source when
  (re)seeding the in-memory pseudo random number generator (PRNG) pool. The
  default of 32 bytes (256 bits) is good enough for 128bit symmetric keys. If
  using EGD or a device file, a maximum of 255 bytes is read.
This feature is available in Postfix 2.2 and later.
Name of the pseudo random number generator (PRNG) state file that is maintained
  by tlsmgr(8). The file is created when it does not exist, and its
  length is fixed at 1024 bytes.
As of version 2.5, Postfix no longer uses root privileges when
    opening this file, and the default file location was changed from
    ${config_directory}/prng_exch to ${data_directory}/prng_exch. As a migration
    aid, an attempt to open the file under a non-Postfix directory is redirected
    to the Postfix-owned data_directory, and a warning is logged.
This feature is available in Postfix 2.2 and later.
The time between attempts by tlsmgr(8) to save the state of the pseudo
  random number generator (PRNG) to the file specified with
  $tls_random_exchange_name.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.2 and later.
The maximal time between attempts by tlsmgr(8) to re-seed the in-memory
  pseudo random number generator (PRNG) pool from external sources. The actual
  time between re-seeding attempts is calculated using the PRNG, and is between
  0 and the time specified.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.2 and later.
The external entropy source for the in-memory tlsmgr(8) pseudo random
  number generator (PRNG) pool. Be sure to specify a non-blocking source. If
  this source is not a regular file, the entropy source type must be prepended:
  egd:/path/to/egd_socket for a source with EGD compatible socket interface, or
  dev:/path/to/device for a device file.
Note: on OpenBSD systems specify dev:/dev/arandom when
    dev:/dev/urandom gives timeout errors.
This feature is available in Postfix 2.2 and later.
Optional lookup tables that map names received from remote SMTP clients via the
  TLS Server Name Indication (SNI) extension to the appropriate keys and
  certificate chains. This parameter is implemented in the Postfix TLS library,
  and applies to both smtpd(8) and the SMTP server mode of
  tlsproxy(8).
When this parameter is non-empty, the Postfix SMTP server enables
    SNI extension processing, and logs SNI values that are invalid or don't
    match an entry in the specified tables. When an entry does match, the SNI
    name is logged as part of the connection summary at log levels 1 and
  higher.
The lookup key is either the verbatim SNI domain name or an
    ancestor domain prefixed with a leading dot. For internationalized domains,
    the lookup key must be in IDNA 2008 A-label form (as required in the TLS SNI
    extension).
The syntax of the lookup value is the same as with the
    smtp_tls_chain_files parameter (see there for additional details), but here
    scoped to just TLS connections in which the client sends a matching SNI
    domain name.
Example:
  
/etc/postfix/main.cf:
    #
    # The indexed SNI table must be created with "postmap -F"
    #
    indexed = ${default_database_type}:${config_directory}/
    tls_server_sni_maps = ${indexed}sni
  
/etc/postfix/sni:
    #
    # The example.com domain has both an RSA and ECDSA certificate
    # chain.  The chain files MUST start with the private key,
    # with the certificate chain next, starting with the leaf
    # (server) certificate, and then the issuer certificates.
    #
    example.com /etc/postfix/sni-chains/rsa2048.example.com.pem,
                /etc/postfix/sni-chains/ecdsa-p256.example.com.pem
    #
    # The example.net domain has a wildcard certificate, and two
    # additional DNS names.  So its certificate chain is also used
    # with any subdomain, plus the additional names.
    #
    example.net /etc/postfix/sni-chains/example.net.pem
    .example.net /etc/postfix/sni-chains/example.net.pem
    example.info /etc/postfix/sni-chains/example.net.pem
    example.org /etc/postfix/sni-chains/example.net.pem
Note that the SNI lookup tables should also have entries for the
    domains that correspond to the Postfix SMTP server's default certificate(s).
    This ensures that the remote SMTP client's TLS SNI extension gets a positive
    response when it specifies one of the Postfix SMTP server's default domains,
    and ensures that the Postfix SMTP server will not log an SNI name mismatch
    for such a domain. The Postfix SMTP server's default certificates are then
    only used when the client sends no SNI or when it sends SNI with a domain
    that the server knows no certificate(s) for.
The mapping from an SNI domain name to a certificate chain is
    indirect. In the input source files for "cdb", "hash",
    "btree" or other tables that are converted to on-disk indexed
    files via postmap(1), the value specified for each key is a list of
    filenames. When postmap(1) is used with the -F option, the
    generated table stores for each lookup key the base64-encoded contents of
    the associated files. When querying tables via postmap -Fq, the table
    value is decoded from base64, yielding the original file content, plus a new
    line.
With "regexp", "pcre", "inline",
    "texthash", "static" and similar tables that are
    interpreted at run-time, and don't have a separate source format, the table
    value is again a list files, that are loaded into memory when the table is
    opened.
With tables whose content is managed outside of Postfix, such as
    LDAP, MySQL, PostgreSQL, socketmap and tcp, the value must be a
    concatenation of the desired PEM keys and certificate chains, that is then
    further encoded to yield a single-line base64 string. Creation of such
    tables and secure storage (the value includes private key material) are
    outside the responsibility of Postfix.
With "socketmap" and "tcp" the data will be
    transmitted in the clear, and there is no query access control, so these are
    generally unsuitable for storing SNI chains. With LDAP and SQL, you should
    restrict read access and use TLS to protect the sensitive data in
  transit.
Typically there is only one private key and its chain of
    certificates starting with the "leaf" certificate corresponding to
    that key, and continuing with the appropriate intermediate issuer CA
    certificates, with each certificate ideally followed by its issuer. Servers
    that have keys and certificates for more than one algorithm (e.g. both an
    RSA key and an ECDSA key, or even RSA, ECDSA and Ed25519) can use multiple
    chains concatenated together, with the key always listed before the
    corresponding certificates.
This feature is available in Postfix 3.4 and later.
Algorithm used to encrypt RFC5077 TLS session tickets. This algorithm must use
  CBC mode, have a 128-bit block size, and must have a key length between 128
  and 256 bits. The default is aes-256-cbc. Overriding the default to choose a
  different algorithm is discouraged.
Setting this parameter empty disables session ticket support in
    the Postfix SMTP server. Another way to disable session ticket support is
    via the tls_ssl_options parameter.
This feature is available in Postfix 3.0 and later.
List or bit-mask of OpenSSL options to enable.
The OpenSSL toolkit provides a set of options that applications
    can enable to tune the OpenSSL behavior. Some of these work around bugs in
    other implementations and are on by default. You can use the
    tls_disable_workarounds parameter to selectively disable some or all of the
    bug work-arounds, making OpenSSL more strict at the cost of
    non-interoperability with SSL clients or servers that exhibit the bugs.
Other options are off by default, and typically enable or disable
    features rather than bug work-arounds. These may be turned on (with care)
    via the tls_ssl_options parameter. The value is a white-space or comma
    separated list of named options chosen from the list below. The names are
    not case-sensitive, you can use lower-case if you prefer. The upper case
    values below match the corresponding macro name in the ssl.h header file
    with the SSL_OP_ prefix removed. It is possible that your OpenSSL version
    includes new options added after your Postfix source code was last updated,
    in that case you can only enable one of these via the hexadecimal syntax
    below.
You should only enable features via the hexadecimal mask when the
    need to control the feature is critical (to deal with a new vulnerability or
    a serious interoperability problem). Postfix DOES NOT promise backwards
    compatible behavior with respect to the mask bits. A feature enabled via the
    mask in one release may be enabled by other means in a later release, and
    the mask bit will then be ignored. Therefore, use of the hexadecimal mask is
    only a temporary measure until a new Postfix or OpenSSL release provides a
    better solution.
If the value of the parameter is a hexadecimal long integer
    starting with "0x", the options corresponding to the bits
    specified in its value are enabled (see openssl/ssl.h and
    SSL_CTX_set_options(3)). You can only enable options not already
    controlled by other Postfix settings. For example, you cannot disable
    protocols or enable server cipher preference. Do not attempt to enable all
    features by specifying 0xFFFFFFFF, this is unlikely to be a good idea. Some
    bug work-arounds are also valid here, allowing them to be re-enabled if/when
    they're no longer enabled by default. The supported values include:
  - ENABLE_MIDDLEBOX_COMPAT
- Postfix >= 3.4. See SSL_CTX_set_options(3).
    
 
- LEGACY_SERVER_CONNECT
- See SSL_CTX_set_options(3).
    
 
- NO_TICKET
- Enabled by default when needed in fully-patched Postfix >= 2.7. Not
      needed at all for Postfix >= 2.11, unless for some reason you do not
      want to support TLS session resumption. Best not set explicitly. See
      SSL_CTX_set_options(3).
    
 
- NO_COMPRESSION
- Disable SSL compression even if supported by the OpenSSL library.
      Compression is CPU-intensive, and compression before encryption does not
      always improve security.
    
 
- NO_RENEGOTIATION
- Postfix >= 3.4. This can reduce opportunities for a potential CPU
      exhaustion attack. See SSL_CTX_set_options(3).
    
 
- NO_SESSION_RESUMPTION_ON_RENEGOTIATION
- Postfix >= 3.4. See SSL_CTX_set_options(3).
    
 
- PRIORITIZE_CHACHA
- Postfix >= 3.4. See SSL_CTX_set_options(3).
    
 
This feature is available in Postfix 2.11 and later.
Match multiple DNS labels with "*" in wildcard certificates.
Some mail service providers prepend the customer domain name to a
    base domain for which they have a wildcard TLS certificate. For example, the
    MX records for example.com hosted by example.net may be:
  
example.com. IN MX 0 example.com.mx1.example.net.
example.com. IN MX 0 example.com.mx2.example.net.
and the TLS certificate may be for "*.example.net". The
    "*" then corresponds with multiple labels in the mail server
    domain name. While multi-label wildcards are not widely supported, and are
    not blessed by any standard, there is little to be gained by disallowing
    their use in this context.
Notes:
  - In a certificate name, the "*" is special only when it is used
      as the first label.
- While Postfix (2.11 or later) can match "*" with multiple domain
      name labels, other implementations likely will not.
- Earlier Postfix implementations behave as if
      "tls_wildcard_matches_multiple_labels = no".
    
 
This feature is available in Postfix 2.11 and later.
The name of the tlsmgr(8) service entry in master.cf. This service
  maintains TLS session caches and other information in support of TLS.
This feature is available in Postfix 2.11 and later.
A file containing CA certificates of root CAs trusted to sign either remote TLS
  server certificates or intermediate CA certificates. See smtp_tls_CAfile for
  further details.
This feature is available in Postfix 3.4 and later.
Directory with PEM format Certification Authority certificates that the Postfix
  tlsproxy(8) client uses to verify a remote TLS server certificate. See
  smtp_tls_CApath for further details.
This feature is available in Postfix 3.4 and later.
File with the Postfix tlsproxy(8) client RSA certificate in PEM format.
  See smtp_tls_cert_file for further details. The preferred way to configure
  tlsproxy client keys and certificates is via the
  "tlsproxy_client_chain_files" parameter.
This feature is available in Postfix 3.4 and later.
Files with the Postfix tlsproxy(8) client keys and certificate chains in
  PEM format. See smtp_tls_chain_files for further details.
This feature is available in Postfix 3.4 and later.
File with the Postfix tlsproxy(8) client DSA certificate in PEM format.
  See smtp_tls_dcert_file for further details. DSA is obsolete and should not be
  used.
This feature is available in Postfix 3.4 and later.
File with the Postfix tlsproxy(8) client DSA private key in PEM format.
  See smtp_tls_dkey_file for further details. DSA is obsolete and should not be
  used.
This feature is available in Postfix 3.4 and later.
File with the Postfix tlsproxy(8) client ECDSA certificate in PEM format.
  See smtp_tls_eccert_file for further details. The preferred way to configure
  tlsproxy client keys and certificates is via the
  "tlsproxy_client_chain_files" parameter.
This feature is available in Postfix 3.4 and later.
File with the Postfix tlsproxy(8) client ECDSA private key in PEM format.
  See smtp_tls_eckey_file for further details. The preferred way to configure
  tlsproxy client keys and certificates is via the
  "tlsproxy_client_chain_files" parameter.
This feature is available in Postfix 3.4 and later.
Enforcement mode: require that SMTP servers use TLS encryption. See
  smtp_enforce_tls for further details. Use tlsproxy_client_security_level
  instead.
This feature is available in Postfix 3.4 and later.
The message digest algorithm used to construct remote TLS server certificate
  fingerprints. See smtp_tls_fingerprint_digest for further details.
This feature is available in Postfix 3.4 and later.
File with the Postfix tlsproxy(8) client RSA private key in PEM format.
  See smtp_tls_key_file for further details. The preferred way to configure
  tlsproxy client keys and certificates is via the
  "tlsproxy_client_chain_files" parameter.
This feature is available in Postfix 3.4 and later.
The default TLS security level for the Postfix tlsproxy(8) client. See
  smtp_tls_security_level for further details.
This feature is available in Postfix 3.4 - 3.6. It was renamed to
    tlsproxy_client_security_level in Postfix 3.7.
Enable additional Postfix tlsproxy(8) client logging of TLS activity. See
  smtp_tls_loglevel for further details.
This feature is available in Postfix 3.4 and later.
The name of the parameter that provides the tlsproxy_client_loglevel value.
This feature is available in Postfix 3.4 and later.
Optional lookup tables with the Postfix tlsproxy(8) client TLS usage
  policy by next-hop destination and by remote TLS server hostname. See
  smtp_tls_per_site for further details.
This feature is available in Postfix 3.4 and later.
Optional lookup tables with the Postfix tlsproxy(8) client TLS security
  policy by next-hop destination. See smtp_tls_policy_maps for further details.
This feature is available in Postfix 3.4 - 3.6. It was renamed to
    tlsproxy_client_policy_maps in Postfix 3.7.
Optional lookup tables with the Postfix tlsproxy(8) client TLS security
  policy by next-hop destination. See smtp_tls_policy_maps for further details.
This feature is available in Postfix 3.7 and later. It was
    previously called tlsproxy_client_policy.
The verification depth for remote TLS server certificates. See
  smtp_tls_scert_verifydepth for further details.
This feature is available in Postfix 3.4 and later.
The default TLS security level for the Postfix tlsproxy(8) client. See
  smtp_tls_security_level for further details.
This feature is available in Postfix 3.7 and later. It was
    previously called tlsproxy_client_level.
Opportunistic mode: use TLS when a remote server announces TLS support. See
  smtp_use_tls for further details. Use tlsproxy_client_security_level instead.
This feature is available in Postfix 3.4 and later.
Mandatory TLS: announce STARTTLS support to remote SMTP clients, and require
  that clients use TLS encryption. See smtpd_enforce_tls for further details.
  Use tlsproxy_tls_security_level instead.
This feature is available in Postfix 2.8 and later.
The name of the tlsproxy(8) service entry in master.cf. This service
  performs plaintext <=> TLS ciphertext conversion.
This feature is available in Postfix 2.8 and later.
A file containing (PEM format) CA certificates of root CAs trusted to sign
  either remote SMTP client certificates or intermediate CA certificates. See
  smtpd_tls_CAfile for further details.
This feature is available in Postfix 2.8 and later.
A directory containing (PEM format) CA certificates of root CAs trusted to sign
  either remote SMTP client certificates or intermediate CA certificates. See
  smtpd_tls_CApath for further details.
This feature is available in Postfix 2.8 and later.
Force the Postfix tlsproxy(8) server to issue a TLS session id, even when
  TLS session caching is turned off. See smtpd_tls_always_issue_session_ids for
  further details.
This feature is available in Postfix 2.8 and later.
Ask a remote SMTP client for a client certificate. See smtpd_tls_ask_ccert for
  further details.
This feature is available in Postfix 2.8 and later.
The verification depth for remote SMTP client certificates. A depth of 1 is
  sufficient if the issuing CA is listed in a local CA file. See
  smtpd_tls_ccert_verifydepth for further details.
This feature is available in Postfix 2.8 and later.
File with the Postfix tlsproxy(8) server RSA certificate in PEM format.
  This file may also contain the Postfix tlsproxy(8) server private RSA
  key. See smtpd_tls_cert_file for further details. With Postfix >= 3.4 the
  preferred way to configure tlsproxy server keys and certificates is via the
  "tlsproxy_tls_chain_files" parameter.
This feature is available in Postfix 2.8 and later.
Files with the Postfix tlsproxy(8) server keys and certificate chains in
  PEM format. See smtpd_tls_chain_files for further details.
This feature is available in Postfix 3.4 and later.
The minimum TLS cipher grade that the Postfix tlsproxy(8) server will use
  with opportunistic TLS encryption. See smtpd_tls_ciphers for further details.
This feature is available in Postfix 2.8 and later.
File with the Postfix tlsproxy(8) server DSA certificate in PEM format.
  This file may also contain the Postfix tlsproxy(8) server private DSA
  key. DSA is obsolete and should not be used. See smtpd_tls_dcert_file for
  further details.
This feature is available in Postfix 2.8 and later.
File with DH parameters that the Postfix tlsproxy(8) server should use
  with non-export EDH ciphers. See smtpd_tls_dh1024_param_file for further
  details.
This feature is available in Postfix 2.8 and later.
File with DH parameters that the Postfix tlsproxy(8) server should use
  with export-grade EDH ciphers. See smtpd_tls_dh512_param_file for further
  details. The default SMTP server cipher grade is "medium" with
  Postfix releases after the middle of 2015, and as a result export-grade cipher
  suites are by default not used.
With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is
    no longer supported, and this parameter is silently ignored.
This feature is available in Postfix 2.8 and later.
File with the Postfix tlsproxy(8) server DSA private key in PEM format.
  This file may be combined with the Postfix tlsproxy(8) server DSA
  certificate file specified with $smtpd_tls_dcert_file. DSA is obsolete and
  should not be used. See smtpd_tls_dkey_file for further details.
This feature is available in Postfix 2.8 and later.
File with the Postfix tlsproxy(8) server ECDSA certificate in PEM format.
  This file may also contain the Postfix tlsproxy(8) server private ECDSA
  key. See smtpd_tls_eccert_file for further details. With Postfix >= 3.4 the
  preferred way to configure tlsproxy server keys and certificates is via the
  "tlsproxy_tls_chain_files" parameter.
This feature is available in Postfix 2.8 and later.
File with the Postfix tlsproxy(8) server ECDSA private key in PEM format.
  This file may be combined with the Postfix tlsproxy(8) server ECDSA
  certificate file specified with $smtpd_tls_eccert_file. See
  smtpd_tls_eckey_file for further details. With Postfix >= 3.4 the preferred
  way to configure tlsproxy server keys and certificates is via the
  "tlsproxy_tls_chain_files" parameter.
This feature is available in Postfix 2.8 and later.
The Postfix tlsproxy(8) server security grade for ephemeral
  elliptic-curve Diffie-Hellman (EECDH) key exchange. See smtpd_tls_eecdh_grade
  for further details.
This feature is available in Postfix 2.8 and later.
List of ciphers or cipher types to exclude from the tlsproxy(8) server
  cipher list at all TLS security levels. See smtpd_tls_exclude_ciphers for
  further details.
This feature is available in Postfix 2.8 and later.
The message digest algorithm to construct remote SMTP client-certificate
  fingerprints. See smtpd_tls_fingerprint_digest for further details.
This feature is available in Postfix 2.8 and later.
File with the Postfix tlsproxy(8) server RSA private key in PEM format.
  This file may be combined with the Postfix tlsproxy(8) server RSA
  certificate file specified with $smtpd_tls_cert_file. See smtpd_tls_key_file
  for further details. With Postfix >= 3.4 the preferred way to configure
  tlsproxy server keys and certificates is via the
  "tlsproxy_tls_chain_files" parameter.
This feature is available in Postfix 2.8 and later.
Enable additional Postfix tlsproxy(8) server logging of TLS activity.
  Each logging level also includes the information that is logged at a lower
  logging level. See smtpd_tls_loglevel for further details.
This feature is available in Postfix 2.8 and later.
The minimum TLS cipher grade that the Postfix tlsproxy(8) server will use
  with mandatory TLS encryption. See smtpd_tls_mandatory_ciphers for further
  details.
This feature is available in Postfix 2.8 and later.
Additional list of ciphers or cipher types to exclude from the
  tlsproxy(8) server cipher list at mandatory TLS security levels. See
  smtpd_tls_mandatory_exclude_ciphers for further details.
This feature is available in Postfix 2.8 and later.
The SSL/TLS protocols accepted by the Postfix tlsproxy(8) server with
  mandatory TLS encryption. If the list is empty, the server supports all
  available SSL/TLS protocol versions. See smtpd_tls_mandatory_protocols for
  further details.
This feature is available in Postfix 2.8 and later.
List of TLS protocols that the Postfix tlsproxy(8) server will exclude or
  include with opportunistic TLS encryption. See smtpd_tls_protocols for further
  details.
This feature is available in Postfix 2.8 and later.
With mandatory TLS encryption, require a trusted remote SMTP client certificate
  in order to allow TLS connections to proceed. See smtpd_tls_req_ccert for
  further details.
This feature is available in Postfix 2.8 and later.
The SMTP TLS security level for the Postfix tlsproxy(8) server; when a
  non-empty value is specified, this overrides the obsolete parameters
  smtpd_use_tls and smtpd_enforce_tls. See smtpd_tls_security_level for further
  details.
This feature is available in Postfix 2.8 and later.
Obsolete expiration time of Postfix tlsproxy(8) server TLS session cache
  information. Since the cache is shared with smtpd(8) and managed by
  tlsmgr(8), there is only one expiration time for the SMTP server cache
  shared by all three services, namely smtpd_tls_session_cache_timeout.
This feature is available in Postfix 2.8 and later.
Opportunistic TLS: announce STARTTLS support to remote SMTP clients, but do not
  require that clients use TLS encryption. See smtpd_use_tls for further
  details. Use tlsproxy_tls_security_level instead.
This feature is available in Postfix 2.8 and later.
How much time a tlsproxy(8) process may take to process local or remote
  I/O before it is terminated by a built-in watchdog timer. This is a safety
  mechanism that prevents tlsproxy(8) from becoming non-responsive due to
  a bug in Postfix itself or in system software. To avoid false alarms and
  unnecessary cache corruption this limit cannot be set under 10s.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
This feature is available in Postfix 2.8 and later
The name of the trace service. This service is implemented by the
  bounce(8) daemon and maintains a record of mail deliveries and produces
  a mail delivery report when verbose delivery is requested with
  "sendmail -v".
This feature is available in Postfix 2.1 and later.
A transport-specific override for the default_delivery_slot_cost parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: transport_delivery_slot_cost parameters will not show
    up in "postconf" command output before Postfix version 2.9. This
    limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_delivery_slot_cost").
A transport-specific override for the default_delivery_slot_discount parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: transport_delivery_slot_discount parameters will not
    show up in "postconf" command output before Postfix version 2.9.
    This limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_delivery_slot_discount").
A transport-specific override for the default_delivery_slot_loan parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: transport_delivery_slot_loan parameters will not show
    up in "postconf" command output before Postfix version 2.9. This
    limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_delivery_slot_loan").
A transport-specific override for the
  default_destination_concurrency_failed_cohort_limit parameter value, where
  transport is the master.cf name of the message delivery transport.
Note: some
    transport_destination_concurrency_failed_cohort_limit parameters will
    not show up in "postconf" command output before Postfix version
    2.9. This limitation applies to many parameters whose name is a combination
    of a master.cf service name and a built-in suffix (in this case:
    "_destination_concurrency_failed_cohort_limit").
This feature is available in Postfix 2.5 and later.
A transport-specific override for the default_destination_concurrency_limit
  parameter value, where transport is the master.cf name of the message
  delivery transport.
Note: some transport_destination_concurrency_limit
    parameters will not show up in "postconf" command output before
    Postfix version 2.9. This limitation applies to many parameters whose name
    is a combination of a master.cf service name and a built-in suffix (in this
    case: "_destination_concurrency_limit").
A transport-specific override for the
  default_destination_concurrency_negative_feedback parameter value, where
  transport is the master.cf name of the message delivery transport.
Note: some
    transport_destination_concurrency_negative_feedback parameters will
    not show up in "postconf" command output before Postfix version
    2.9. This limitation applies to many parameters whose name is a combination
    of a master.cf service name and a built-in suffix (in this case:
    "_destination_concurrency_negative_feedback").
This feature is available in Postfix 2.5 and later.
A transport-specific override for the
  default_destination_concurrency_positive_feedback parameter value, where
  transport is the master.cf name of the message delivery transport.
Note: some
    transport_destination_concurrency_positive_feedback parameters will
    not show up in "postconf" command output before Postfix version
    2.9. This limitation applies to many parameters whose name is a combination
    of a master.cf service name and a built-in suffix (in this case:
    "_destination_concurrency_positive_feedback").
This feature is available in Postfix 2.5 and later.
A transport-specific override for the default_destination_rate_delay parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: some transport_destination_rate_delay parameters will
    not show up in "postconf" command output before Postfix version
    2.9. This limitation applies to many parameters whose name is a combination
    of a master.cf service name and a built-in suffix (in this case:
    "_destination_rate_delay").
This feature is available in Postfix 2.5 and later.
A transport-specific override for the default_destination_recipient_limit
  parameter value, where transport is the master.cf name of the message
  delivery transport.
Note: some transport_destination_recipient_limit parameters
    will not show up in "postconf" command output before Postfix
    version 2.9. This limitation applies to many parameters whose name is a
    combination of a master.cf service name and a built-in suffix (in this case:
    "_destination_recipient_limit").
A transport-specific override for the default_extra_recipient_limit parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: transport_extra_recipient_limit parameters will not
    show up in "postconf" command output before Postfix version 2.9.
    This limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_extra_recipient_limit").
A transport-specific override for the initial_destination_concurrency parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: some transport_initial_destination_concurrency
    parameters will not show up in "postconf" command output before
    Postfix version 2.9. This limitation applies to many parameters whose name
    is a combination of a master.cf service name and a built-in suffix (in this
    case: "_initial_destination_concurrency").
This feature is available in Postfix 2.5 and later.
Optional lookup tables with mappings from recipient address to (message delivery
  transport, next-hop destination). See transport(5) for details.
Specify zero or more "type:table" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found. If you use this feature with local files, run
    "postmap /etc/postfix/transport" after making a change.
Pattern matching of domain names is controlled by the presence or
    absence of "transport_maps" in the
    parent_domain_matches_subdomains parameter value.
For safety reasons, as of Postfix 2.3 this feature does not allow
    $number substitutions in regular expression maps.
Examples:
transport_maps = dbm:/etc/postfix/transport
transport_maps = hash:/etc/postfix/transport
A transport-specific override for the default_minimum_delivery_slots parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: transport_minimum_delivery_slots parameters will not
    show up in "postconf" command output before Postfix version 2.9.
    This limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_minimum_delivery_slots").
A transport-specific override for the default_recipient_limit parameter value,
  where transport is the master.cf name of the message delivery
  transport.
Note: some transport_recipient_limit parameters will not
    show up in "postconf" command output before Postfix version 2.9.
    This limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_recipient_limit").
A transport-specific override for the default_recipient_refill_delay parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: transport_recipient_refill_delay parameters will not
    show up in "postconf" command output before Postfix version 2.9.
    This limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_recipient_refill_delay").
This feature is available in Postfix 2.4 and later.
A transport-specific override for the default_recipient_refill_limit parameter
  value, where transport is the master.cf name of the message delivery
  transport.
Note: transport_recipient_refill_limit parameters will not
    show up in "postconf" command output before Postfix version 2.9.
    This limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_recipient_refill_limit").
This feature is available in Postfix 2.4 and later.
The time between attempts by the Postfix queue manager to contact a
  malfunctioning message delivery transport.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
A transport-specific override for the command_time_limit parameter value, where
  transport is the master.cf name of the message delivery transport.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
Note: transport_time_limit parameters will not show up in
    "postconf" command output before Postfix version 2.9. This
    limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_time_limit").
A transport-specific override for the default_transport_rate_delay parameter
  value, where the initial transport in the parameter name is the
  master.cf name of the message delivery transport.
Specify a non-negative time value (an integral value plus an
    optional one-letter suffix that specifies the time unit). Time units: s
    (seconds), m (minutes), h (hours), d (days), w (weeks). The default time
    unit is s (seconds).
Note: transport_transport_rate_delay parameters will not
    show up in "postconf" command output before Postfix version 2.9.
    This limitation applies to many parameters whose name is a combination of a
    master.cf service name and a built-in suffix (in this case:
    "_transport_rate_delay").
The time limit for sending a trigger to a Postfix daemon (for example, the
  pickup(8) or qmgr(8) daemon). This time limit prevents programs
  from getting stuck when the mail system is under heavy load.
Specify a non-zero time value (an integral value plus an optional
    one-letter suffix that specifies the time unit). Time units: s (seconds), m
    (minutes), h (hours), d (days), w (weeks). The default time unit is s
    (seconds).
Message header that the Postfix cleanup(8) server inserts when a message
  contains no To: or Cc: message header. With Postfix 2.8 and later, the default
  value is empty. With Postfix 2.4-2.7, specify an empty value to disable this
  feature.
Example:
# Default value before Postfix 2.8.
# Note: the ":" and ";" are both required.
undisclosed_recipients_header = To: undisclosed-recipients:;
The numerical response code when the Postfix SMTP server rejects a sender or
  recipient address because its domain is unknown. This is one of the possible
  replies from the restrictions reject_unknown_sender_domain and
  reject_unknown_recipient_domain.
Do not change this unless you have a complete understanding of RFC
    5321.
The Postfix SMTP server's action when reject_unknown_sender_domain or
  reject_unknown_recipient_domain fail due to a temporary error condition.
  Specify "defer" to defer the remote SMTP client request immediately.
  With the default "defer_if_permit" action, the Postfix SMTP server
  continues to look for opportunities to reject mail, and defers the client
  request only if it would otherwise be accepted.
This feature is available in Postfix 2.6 and later.
The numerical Postfix SMTP server response code when a client without valid
  address <=> name mapping is rejected by the
  reject_unknown_client_hostname restriction. The SMTP server always replies
  with 450 when the mapping failed due to a temporary error condition.
Do not change this unless you have a complete understanding of RFC
    5321.
The Postfix SMTP server's action when reject_unknown_helo_hostname fails due to
  a temporary error condition. Specify "defer" to defer the remote
  SMTP client request immediately. With the default "defer_if_permit"
  action, the Postfix SMTP server continues to look for opportunities to reject
  mail, and defers the client request only if it would otherwise be accepted.
This feature is available in Postfix 2.6 and later.
The numerical Postfix SMTP server response code when the hostname specified with
  the HELO or EHLO command is rejected by the reject_unknown_helo_hostname
  restriction.
Do not change this unless you have a complete understanding of RFC
    5321.
The numerical Postfix SMTP server response code when a recipient address is
  local, and $local_recipient_maps specifies a list of lookup tables that does
  not match the recipient. A recipient address is local when its domain matches
  $mydestination, $proxy_interfaces or $inet_interfaces.
The default setting is 550 (reject mail) but it is safer to
    initially use 450 (try again later) so you have time to find out if your
    local_recipient_maps settings are OK.
Example:
unknown_local_recipient_reject_code = 450
This feature is available in Postfix 2.0 and later.
The numerical Postfix SMTP server reply code when a recipient address matches
  $relay_domains, and relay_recipient_maps specifies a list of lookup tables
  that does not match the recipient address.
This feature is available in Postfix 2.0 and later.
The Postfix SMTP server reply code when a recipient address matches
  $virtual_alias_domains, and $virtual_alias_maps specifies a list of lookup
  tables that does not match the recipient address.
This feature is available in Postfix 2.0 and later.
The Postfix SMTP server reply code when a recipient address matches
  $virtual_mailbox_domains, and $virtual_mailbox_maps specifies a list of lookup
  tables that does not match the recipient address.
This feature is available in Postfix 2.0 and later.
The numerical Postfix SMTP server response when a recipient address probe fails
  due to a temporary error condition.
Unlike elsewhere in Postfix, you can specify 250 in order to
    accept the address anyway.
Do not change this unless you have a complete understanding of RFC
    5321.
This feature is available in Postfix 2.6 and later.
The numerical Postfix SMTP server response when a recipient address is rejected
  by the reject_unverified_recipient restriction.
Unlike elsewhere in Postfix, you can specify 250 in order to
    accept the address anyway.
Do not change this unless you have a complete understanding of RFC
    5321.
This feature is available in Postfix 2.1 and later.
The Postfix SMTP server's reply when rejecting mail with
  reject_unverified_recipient. Do not include the numeric SMTP reply code or the
  enhanced status code. By default, the response includes actual address
  verification details.
Example:
unverified_recipient_reject_reason = Recipient address lookup failed
This feature is available in Postfix 2.6 and later.
The Postfix SMTP server's action when reject_unverified_recipient fails due to a
  temporary error condition. Specify "defer" to defer the remote SMTP
  client request immediately. With the default "defer_if_permit"
  action, the Postfix SMTP server continues to look for opportunities to reject
  mail, and defers the client request only if it would otherwise be accepted.
This feature is available in Postfix 2.6 and later.
The numerical Postfix SMTP server response code when a sender address probe
  fails due to a temporary error condition.
Unlike elsewhere in Postfix, you can specify 250 in order to
    accept the address anyway.
Do not change this unless you have a complete understanding of RFC
    5321.
This feature is available in Postfix 2.6 and later.
The numerical Postfix SMTP server response code when a recipient address is
  rejected by the reject_unverified_sender restriction.
Unlike elsewhere in Postfix, you can specify 250 in order to
    accept the address anyway.
Do not change this unless you have a complete understanding of RFC
    5321.
This feature is available in Postfix 2.1 and later.
The Postfix SMTP server's reply when rejecting mail with
  reject_unverified_sender. Do not include the numeric SMTP reply code or the
  enhanced status code. By default, the response includes actual address
  verification details.
Example:
unverified_sender_reject_reason = Sender address lookup failed
This feature is available in Postfix 2.6 and later.
The Postfix SMTP server's action when reject_unverified_sender fails due to a
  temporary error condition. Specify "defer" to defer the remote SMTP
  client request immediately. With the default "defer_if_permit"
  action, the Postfix SMTP server continues to look for opportunities to reject
  mail, and defers the client request only if it would otherwise be accepted.
This feature is available in Postfix 2.6 and later.
Enables discovery for the specified service(s) using DNS SRV records. For
  example, with "use_srv_lookup = submission" and "relayhost =
  example.com:submission", the Postfix SMTP client will look up DNS SRV
  records for _submission._tcp.example.com, and will relay email through the
  hosts and ports that are specified with those records. See RFC 2782 for
  details of the host selection process.
Specify zero or more service names separated by comma and/or
    whitespace. Any name in the services(5) database may be specified,
    though in practice only submission, submissions, and smtp make sense.
When SRV record lookup is enabled with use_srv_lookup, you can
    enclose a domain name in "[]" to force IP address lookup instead
    of SRV record lookup.
Example 1: MUA-to-MTA submission using SRV record lookup for the
    "submission" service for domain "example.com". This uses
    the default SMTP delivery agent with STARTTLS, and looks up SRV records for
    "_submission._tcp.example.com".
/etc/postfix/main.cf:
    use_srv_lookup = submission
    relayhost = example.com:submission
    smtp_tls_security_level = may
    ...see SASL_README for sasl configuration...
Example 2: MUA-to-MTA submission using SRV record lookup for the
    "submissions" service for domain "example.org". This
    uses a dedicated SMTP delivery agent (smtp-wraptls) with tls_wrappermode
    turned on, and looks up SRV records for
    "_submissions._tcp.example.org".
Note: specify the older name "smtps" instead of
    "submissions" when a provider has DNS SRV records like
    "_smtps._tcp.example.org" instead of
    "_submissions._tcp.example.org".
/etc/postfix/main.cf:
    use_srv_lookup = submissions
    default_transport = smtp-wraptls:example.org:submissions
    ...see SASL_README for sasl configuration...
/etc/postfix/master.cf:
    smtp-wraptls   unix   ...   ...   ...   ...   ...   smtp
        -o { smtp_tls_wrappermode = yes }
        -o { smtp_tls_security_level = encrypt }
Example 3: Sender-dependent selection for a combination of
    MUA-to-MTA submission services. This combines examples 1 and 2 with examples
    of how to disable SRV and look up IP address records for
    "smtp-relay.example.net" and "smtp-relay.other.example".
    Again, specify the older name "smtps" instead of
    "submissions" when a provider has DNS SRV records like
    "_smtps._tcp.example.org" instead of
    "_submissions._tcp.example.org".
/etc/postfix/main.cf:
    use_srv_lookup = submission, submissions
    sender_dependent_default_transport_maps = inline:{
        # Destinations that support SRV record lookup.
        { user1@example.com = smtp:example.com:submission }
        { user2@example.org = smtp-wraptls:example.org:submissions }
        # Use [destination] to force IP address lookups.
        { user3@example.net = smtp:[smtp-relay.example.net]:submission }
        { user4@other.example =
              smtp-wraptls:[smtp-relay.other.example]:submissions } }
    ...see SASL_README for sasl configuration...
Example 4: MTA-to-MTA traffic, using SRV record lookup for the
    SMTP service. This is useful for Postfix tests, and may be useful in
    environments where ports are dynamically assigned to servers.
/etc/postfix/main.cf:
    use_srv_lookup = smtp
    # Fall back to MX record lookup when SRV records are unavailable.
    #allow_srv_lookup_fallback = yes
    #ignore_srv_lookup_error = yes
This feature is available in Postfix 3.8 and later.
The characters Postfix accepts as VERP delimiter characters on the Postfix
  sendmail(1) command line and in SMTP commands.
This feature is available in Postfix 1.1 and later.
The maximal length of an email address after virtual alias expansion. This stops
  virtual aliasing loops that increase the address length exponentially.
This feature is available in Postfix 3.0 and later.
Postfix is the final destination for the specified list of virtual alias
  domains, that is, domains for which all addresses are aliased to addresses in
  other local or remote domains. The SMTP server validates recipient addresses
  with $virtual_alias_maps and rejects non-existent recipients. See also the
  virtual alias domain class in the ADDRESS_CLASS_README file
This feature is available in Postfix 2.0 and later. The default
    value is backwards compatible with Postfix version 1.1.
The default value is $virtual_alias_maps so that you can keep all
    information about virtual alias domains in one place. If you have many
    users, it is better to separate information that changes more frequently
    (virtual address -> local or remote address mapping) from information
    that changes less frequently (the list of virtual domain names).
Specify a list of host or domain names, "/file/name" or
    "type:table" patterns, separated by commas and/or whitespace. A
    "/file/name" pattern is replaced by its contents; a
    "type:table" lookup table is matched when a table entry matches a
    host or domain name (the lookup result is ignored). Continue long lines by
    starting the next line with whitespace. Specify "!pattern" to
    exclude a host or domain name from the list. The form
    "!/file/name" is supported only in Postfix version 2.4 and
  later.
See also the VIRTUAL_README and ADDRESS_CLASS_README documents for
    further information.
Example:
virtual_alias_domains = virtual1.tld virtual2.tld
The maximal number of addresses that virtual alias expansion produces from each
  original recipient.
This feature is available in Postfix 2.1 and later.
Optional lookup tables that alias specific mail addresses or domains to other
  local or remote addresses. The table format and lookups are documented in
  virtual(5). For an overview of Postfix address manipulations see the
  ADDRESS_REWRITING_README document.
This feature is available in Postfix 2.0 and later. The default
    value is backwards compatible with Postfix version 1.1.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found. Note: these lookups are recursive.
If you use this feature with indexed files, run
    "postmap /etc/postfix/virtual" after changing the
    file.
Examples:
virtual_alias_maps = dbm:/etc/postfix/virtual
virtual_alias_maps = hash:/etc/postfix/virtual
The maximal nesting depth of virtual alias expansion. Currently the recursion
  limit is applied only to the left branch of the expansion graph, so the depth
  of the tree can in the worst case reach the sum of the expansion and recursion
  limits. This may change in the future.
This feature is available in Postfix 2.1 and later.
Optional filter for the virtual(8) delivery agent to change the delivery
  status code or explanatory text of successful or unsuccessful deliveries. See
  default_delivery_status_filter for details.
This feature is available in Postfix 3.0 and later.
The maximal number of parallel deliveries to the same destination via the
  virtual message delivery transport. This limit is enforced by the queue
  manager. The message delivery transport name is the first field in the entry
  in the master.cf file.
The maximal number of recipients per message for the virtual message delivery
  transport. This limit is enforced by the queue manager. The message delivery
  transport name is the first field in the entry in the master.cf file.
Setting this parameter to a value of 1 changes the meaning of
    virtual_destination_concurrency_limit from concurrency per domain into
    concurrency per recipient.
Lookup tables with the per-recipient group ID for virtual(8) mailbox
  delivery.
This parameter is specific to the virtual(8) delivery
    agent. It does not apply when mail is delivered with a different mail
    delivery program.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
In a lookup table, specify a left-hand side of
    "@domain.tld" to match any user in the specified domain that does
    not have a specific "user@domain.tld" entry.
When a recipient address has an optional address extension
    (user+foo@domain.tld), the virtual(8) delivery agent looks up the
    full address first, and when the lookup fails, it looks up the unextended
    address (user@domain.tld).
Note 1: for security reasons, the virtual(8) delivery agent
    disallows regular expression substitution of $1 etc. in regular expression
    lookup tables, because that would open a security hole.
Note 2: for security reasons, the virtual(8) delivery agent
    will silently ignore requests to use the proxymap(8) server. Instead
    it will open the table directly. Before Postfix version 2.2, the
    virtual(8) delivery agent will terminate with a fatal error.
A prefix that the virtual(8) delivery agent prepends to all pathname
  results from $virtual_mailbox_maps table lookups. This is a safety measure to
  ensure that an out of control map doesn't litter the file system with
  mailboxes. While virtual_mailbox_base could be set to "/", this
  setting isn't recommended.
This parameter is specific to the virtual(8) delivery
    agent. It does not apply when mail is delivered with a different mail
    delivery program.
Example:
virtual_mailbox_base = /var/mail
Postfix is the final destination for the specified list of domains; mail is
  delivered via the $virtual_transport mail delivery transport. By default this
  is the Postfix virtual(8) delivery agent. The SMTP server validates
  recipient addresses with $virtual_mailbox_maps and rejects mail for
  non-existent recipients. See also the virtual mailbox domain class in the
  ADDRESS_CLASS_README file.
This parameter expects the same syntax as the mydestination
    configuration parameter.
This feature is available in Postfix 2.0 and later. The default
    value is backwards compatible with Postfix version 1.1.
The maximal size in bytes of an individual virtual(8) mailbox or maildir
  file, or zero (no limit).
This parameter is specific to the virtual(8) delivery
    agent. It does not apply when mail is delivered with a different mail
    delivery program.
How to lock a UNIX-style virtual(8) mailbox before attempting delivery.
  For a list of available file locking methods, use the "postconf
  -l" command.
This parameter is specific to the virtual(8) delivery
    agent. It does not apply when mail is delivered with a different mail
    delivery program.
This setting is ignored with maildir style delivery,
    because such deliveries are safe without application-level locks.
Note 1: the dotlock method requires that the recipient UID
    or GID has write access to the parent directory of the recipient's mailbox
    file.
Note 2: the default setting of this parameter is system
  dependent.
Optional lookup tables with all valid addresses in the domains that match
  $virtual_mailbox_domains.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
In a lookup table, specify a left-hand side of
    "@domain.tld" to match any user in the specified domain that does
    not have a specific "user@domain.tld" entry.
With the default "virtual_mailbox_domains =
    $virtual_mailbox_maps", lookup tables also need entries with a
    left-hand side of "domain.tld" to satisfy virtual_mailbox_domain
    lookups (the right-hand side is required but will not be used).
The remainder of this text is specific to the virtual(8)
    delivery agent. It does not apply when mail is delivered with a different
    mail delivery program.
The virtual(8) delivery agent uses this table to look up
    the per-recipient mailbox or maildir pathname. If the lookup result ends in
    a slash ("/"), maildir-style delivery is carried out, otherwise
    the path is assumed to specify a UNIX-style mailbox file. Note that
    $virtual_mailbox_base is unconditionally prepended to this path.
When a recipient address has an optional address extension
    (user+foo@domain.tld), the virtual(8) delivery agent looks up the
    full address first, and when the lookup fails, it looks up the unextended
    address (user@domain.tld).
Note 1: for security reasons, the virtual(8) delivery agent
    disallows regular expression substitution of $1 etc. in regular expression
    lookup tables, because that would open a security hole.
Note 2: for security reasons, the virtual(8) delivery agent
    will silently ignore requests to use the proxymap(8) server. Instead
    it will open the table directly. Before Postfix version 2.2, the
    virtual(8) delivery agent will terminate with a fatal error.
Optional lookup tables with a) names of domains for which all addresses are
  aliased to addresses in other local or remote domains, and b) addresses that
  are aliased to addresses in other local or remote domains. Available before
  Postfix version 2.0. With Postfix version 2.0 and later, this is replaced by
  separate controls: virtual_alias_domains and virtual_alias_maps.
The minimum user ID value that the virtual(8) delivery agent accepts as a
  result from $virtual_uid_maps table lookup. Returned values less than this
  will be rejected, and the message will be deferred.
This parameter is specific to the virtual(8) delivery
    agent. It does not apply when mail is delivered with a different mail
    delivery program.
The default mail delivery transport and next-hop destination for final delivery
  to domains listed with $virtual_mailbox_domains. This information can be
  overruled with the transport(5) table.
Specify a string of the form transport:nexthop, where
    transport is the name of a mail delivery transport defined in
    master.cf. The :nexthop destination is optional; its syntax is
    documented in the manual page of the corresponding delivery agent.
This feature is available in Postfix 2.0 and later.
Lookup tables with the per-recipient user ID that the virtual(8) delivery
  agent uses while writing to the recipient's mailbox.
This parameter is specific to the virtual(8) delivery
    agent. It does not apply when mail is delivered with a different mail
    delivery program.
Specify zero or more "type:name" lookup tables,
    separated by whitespace or comma. Tables will be searched in the specified
    order until a match is found.
In a lookup table, specify a left-hand side of
    "@domain.tld" to match any user in the specified domain that does
    not have a specific "user@domain.tld" entry.
When a recipient address has an optional address extension
    (user+foo@domain.tld), the virtual(8) delivery agent looks up the
    full address first, and when the lookup fails, it looks up the unextended
    address (user@domain.tld).
Note 1: for security reasons, the virtual(8) delivery agent
    disallows regular expression substitution of $1 etc. in regular expression
    lookup tables, because that would open a security hole.
Note 2: for security reasons, the virtual(8) delivery agent
    will silently ignore requests to use the proxymap(8) server. Instead
    it will open the table directly. Before Postfix version 2.2, the
    virtual(8) delivery agent will terminate with a fatal error.
postconf(1), Postfix configuration parameter maintenance
master(5), Postfix daemon configuration maintenance
The Secure Mailer license must be distributed with this software.
Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA
Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA
Viktor Dukhovni