SMTP(8) System Manager's Manual SMTP(8)

smtp - Postfix SMTP+LMTP client

smtp [generic Postfix daemon options]

The Postfix SMTP+LMTP client implements the SMTP and LMTP mail delivery protocols. It processes message delivery requests from the queue manager. Each request specifies a queue file, a sender address, a domain or host to deliver to, and recipient information. This program expects to be run from the master(8) process manager.

The SMTP+LMTP client updates the queue file and marks recipients as finished, or it informs the queue manager that delivery should be tried again at a later time. Delivery status reports are sent to the bounce(8), defer(8) or trace(8) daemon as appropriate.

The SMTP+LMTP client looks up a list of mail exchanger addresses for the destination host, sorts the list by preference, and connects to each listed address until it finds a server that responds.

When a server is not reachable, or when mail delivery fails due to a recoverable error condition, the SMTP+LMTP client will try to deliver the mail to an alternate host.

After a successful mail transaction, a connection may be saved to the scache(8) connection cache server, so that it may be used by any SMTP+LMTP client for a subsequent transaction.

By default, connection caching is enabled temporarily for destinations that have a high volume of mail in the active queue. Connection caching can be enabled permanently for specific destinations.



SMTP destinations have the following form:



  

  

  

  
Look up the mail exchangers for the specified domain, and connect to the
      specified port (default: smtp).

  
[hostname]

  

  
[hostname]:port

  
Look up the address(es) of the specified host, and connect to the
      specified port (default: smtp).

  
[address]

  

  
[address]:port

  
Connect to the host at the specified address, and connect to the specified
      port (default: smtp). An IPv6 address must be formatted as
      [ipv6:address].












LMTP destinations have the following form:



  

  
Connect to the local UNIX-domain server that is bound to the specified
      pathname. If the process runs chrooted, an absolute pathname is
      interpreted relative to the Postfix queue directory.

  

  

  

  

  

  

  

  
Connect to the specified TCP port on the specified local or remote host.
      If no port is specified, connect to the port defined as lmtp in
      services(4). If no such service is found, the lmtp_tcp_port
      configuration parameter (default value of 24) will be used. An IPv6
      address must be formatted as [ipv6:address].












The SMTP+LMTP client is moderately security-sensitive. It talks to
    SMTP or LMTP servers and to DNS servers on the network. The SMTP+LMTP client
    can be run chrooted at fixed low privilege.








RFC 821 (SMTP protocol)
RFC 822 (ARPA Internet Text Messages)
RFC 1651 (SMTP service extensions)
RFC 1652 (8bit-MIME transport)
RFC 1870 (Message Size Declaration)
RFC 2033 (LMTP protocol)
RFC 2034 (SMTP Enhanced Error Codes)
RFC 2045 (MIME: Format of Internet Message Bodies)
RFC 2046 (MIME: Media Types)
RFC 2554 (AUTH command)
RFC 2821 (SMTP protocol)
RFC 2920 (SMTP Pipelining)
RFC 3207 (STARTTLS command)
RFC 3461 (SMTP DSN Extension)
RFC 3463 (Enhanced Status Codes)
RFC 4954 (AUTH command)
RFC 5321 (SMTP protocol)
RFC 6531 (Internationalized SMTP)
RFC 6533 (Internationalized Delivery Status Notifications)
RFC 7672 (SMTP security via opportunistic DANE TLS)










Problems and transactions are logged to syslogd(8).
    Corrupted message files are marked so that the queue manager can move them
    to the corrupt queue for further inspection.


Depending on the setting of the notify_classes parameter,
    the postmaster is notified of bounces, protocol problems, and of other
    trouble.










SMTP and LMTP connection caching does not work with TLS. The
    necessary support for TLS object passivation and re-activation does not
    exist without closing the session, which defeats the purpose.


SMTP and LMTP connection caching assumes that SASL credentials are
    valid for all destinations that map onto the same IP address and TCP
  port.










Before Postfix version 2.3, the LMTP client is a separate program
    that implements only a subset of the functionality available with SMTP:
    there is no support for TLS, and connections are cached in-process, making
    it ineffective when the client is used for multiple domains.


Most smtp_xxx configuration parameters have an
    lmtp_xxx "mirror" parameter for the equivalent LMTP
    feature. This document describes only those LMTP-related parameters that
    aren't simply "mirror" parameters.


Changes to main.cf are picked up automatically, as
    smtp(8) processes run for only a limited amount of time. Use the
    command "postfix reload" to speed up a change.


The text below provides only a parameter summary. See
    postconf(5) for more details including examples.











  

  
Ignore DNS MX lookups that produce no response.

  

  
Always send EHLO at the start of an SMTP session.

  

  
Never send EHLO at the start of an SMTP session.

  

  
Defer mail delivery when no MX record resolves to an IP address.

  

  
The maximal length of message header and body lines that Postfix will send
      via SMTP.

  

  
How long the Postfix SMTP client pauses before sending
      ".<CR><LF>" in order to work around the PIX firewall
      "<CR><LF>.<CR><LF>" bug.

  

  
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.

  

  
A list that specifies zero or more workarounds for CISCO PIX firewall
      bugs.

  

  
Lookup tables, indexed by the remote SMTP server address, with
      per-destination workarounds for CISCO PIX firewall bugs.

  

  
Quote addresses in Postfix SMTP client MAIL FROM and RCPT TO commands as
      required by RFC 5321.

  

  
A mechanism to transform replies from remote SMTP servers one line at a
      time.

  

  
Skip remote SMTP servers that greet with a 5XX status code.

  

  
Do not wait for the response to the SMTP QUIT command.




Available in Postfix version 2.0 and earlier:



  

  
Skip SMTP servers that greet with a 4XX status code (go away, try again
      later).




Available in Postfix version 2.2 and later:



  

  
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.

  

  
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.

  

  
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.




Available in Postfix version 2.2.9 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.




Available in Postfix version 2.3 and later:



  

  
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.

  

  
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.




Available in Postfix version 2.4.4 and later:



  

  
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.




Available in Postfix version 2.5 and later:



  

  
Restricted header_checks(5) tables for the Postfix SMTP
    client.

  

  
Restricted mime_header_checks(5) tables for the Postfix SMTP
      client.

  

  
Restricted nested_header_checks(5) tables for the Postfix SMTP
      client.

  

  
Restricted body_checks(5) tables for the Postfix SMTP client.




Available in Postfix version 2.6 and later:



  

  
An optional workaround for routers that break TCP window scaling.




Available in Postfix version 2.8 and later:



  

  
DNS Resolver options for the Postfix SMTP client.




Available in Postfix version 2.9 and later:



  

  
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).

  

  
Whether or not to append the "AUTH=<>" option to the MAIL
      FROM command in SASL-authenticated SMTP sessions.




Available in Postfix version 2.11 and later:



  

  
Level of DNS support in the Postfix SMTP client.




Available in Postfix version 3.0 and later:



  

  
Optional filter for the smtp(8) delivery agent to change the
      delivery status code or explanatory text of successful or unsuccessful
      deliveries.

  

  
Optional filter for Postfix SMTP client DNS lookup results.












Available in Postfix version 2.0 and later:



  

  
Disable the conversion of 8BITMIME format to 7BIT format.

  

  
The maximal length of MIME multipart boundary strings.

  

  
The maximal recursion level that the MIME processor will handle.












Available in Postfix version 2.1 and later:



  

  
Send the non-standard XFORWARD command when the Postfix SMTP server EHLO
      response announces XFORWARD support.













  

  
Enable SASL authentication in the Postfix SMTP client.

  

  
Optional Postfix SMTP client lookup tables with one username:password
      entry per sender, remote hostname or next-hop domain.

  

  
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.




Available in Postfix version 2.2 and later:



  

  
If non-empty, a Postfix SMTP client filter for the remote SMTP server's
      list of offered SASL mechanisms.




Available in Postfix version 2.3 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.

  

  
Implementation-specific information that the Postfix SMTP client passes
      through to the SASL plug-in implementation that is selected with
      smtp_sasl_type.

  

  
The SASL plug-in type that the Postfix SMTP client should use for
      authentication.




Available in Postfix version 2.5 and later:



  

  
An optional table to prevent repeated SASL authentication failures with
      the same remote SMTP server hostname, username and password.

  

  
The maximal age of an smtp_sasl_auth_cache_name entry before it is
      removed.

  

  
When a remote SMTP server rejects a SASL authentication request with a 535
      reply code, defer mail delivery instead of returning mail as
      undeliverable.




Available in Postfix version 2.9 and later:



  

  
Whether or not to append the "AUTH=<>" option to the MAIL
      FROM command in SASL-authenticated SMTP sessions.












Detailed information about STARTTLS configuration may be found in
    the TLS_README document.



  

  
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.

  

  
The SASL authentication security options that the Postfix SMTP client uses
      for TLS encrypted SMTP sessions.

  

  
Time limit for Postfix SMTP client write and read operations during TLS
      startup and shutdown handshake procedures.

  

  
A file containing CA certificates of root CAs trusted to sign either
      remote SMTP server certificates or intermediate CA certificates.

  

  
Directory with PEM format Certification Authority certificates that the
      Postfix SMTP client uses to verify a remote SMTP server certificate.

  

  
File with the Postfix SMTP client RSA certificate in PEM format.

  

  
The minimum TLS cipher grade that the Postfix SMTP client will use with
      mandatory TLS encryption.

  

  
List of ciphers or cipher types to exclude from the Postfix SMTP client
      cipher list at all TLS security levels.

  

  
Additional list of ciphers or cipher types to exclude from the Postfix
      SMTP client cipher list at mandatory TLS security levels.

  

  
File with the Postfix SMTP client DSA certificate in PEM format.

  

  
File with the Postfix SMTP client DSA private key in PEM format.

  

  
File with the Postfix SMTP client RSA private key in PEM format.

  

  
Enable additional Postfix SMTP client logging of TLS activity.

  

  
Log the hostname of a remote SMTP server that offers STARTTLS, when TLS is
      not already enabled for that server.

  

  
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.

  

  
List of SSL/TLS protocols that the Postfix SMTP client will use with
      mandatory TLS encryption.

  

  
The verification depth for remote SMTP server certificates.

  

  
How the Postfix SMTP client verifies the server certificate peername for
      the "secure" TLS security level.

  

  
Name of the file containing the optional Postfix SMTP client TLS session
      cache.

  

  
The expiration time of Postfix SMTP client TLS session cache
    information.

  

  
How the Postfix SMTP client verifies the server certificate peername for
      the "verify" TLS security level.

  

  
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 OpenSSL cipherlist for "high" grade ciphers.

  

  
The OpenSSL cipherlist for "medium" or higher grade
    ciphers.

  

  
The OpenSSL cipherlist for "low" or higher grade ciphers.

  

  
The OpenSSL cipherlist for "export" or higher grade
    ciphers.

  

  
The OpenSSL cipherlist for "NULL" grade ciphers that provide
      authentication without encryption.




Available in Postfix version 2.4 and later:



  

  
The SASL authentication security options that the Postfix SMTP client uses
      for TLS encrypted SMTP sessions with a verified server certificate.




Available in Postfix version 2.5 and later:



  

  
List of acceptable remote SMTP server certificate fingerprints for the
      "fingerprint" TLS security level (smtp_tls_security_level
      = fingerprint).

  

  
The message digest algorithm used to construct remote SMTP server
      certificate fingerprints.




Available in Postfix version 2.6 and later:



  

  
List of TLS protocols that the Postfix SMTP client will exclude or include
      with opportunistic TLS encryption.

  

  
The minimum TLS cipher grade that the Postfix SMTP client will use with
      opportunistic TLS encryption.

  

  
File with the Postfix SMTP client ECDSA certificate in PEM format.

  

  
File with the Postfix SMTP client ECDSA private key in PEM format.




Available in Postfix version 2.7 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.




Available in Postfix version 2.8 and later:



  

  
List or bit-mask of OpenSSL bug work-arounds to disable.




Available in Postfix version 2.11 and later:



  

  
Zero or more PEM-format files with trust-anchor certificates and/or public
      keys.

  

  
Lookup the associated DANE TLSA RRset even when a hostname is not an alias
      and its address records lie in an unsigned zone.

  

  
The name of the tlsmgr(8) service entry in master.cf.




Available in Postfix version 3.0 and later:



  

  
Request that the Postfix SMTP client connects using the legacy SMTPS
      protocol instead of using the STARTTLS command.




Available in Postfix version 3.1 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 following configuration parameters exist for compatibility
    with Postfix versions before 2.3. Support for these will be removed in a
    future release.



  

  
Opportunistic mode: use TLS when a remote SMTP server announces STARTTLS
      support, otherwise send the mail in the clear.

  

  
Enforcement mode: require that remote SMTP servers use TLS encryption, and
      never send mail in the clear.

  

  
With mandatory TLS encryption, require that the remote SMTP server
      hostname matches the information in the remote SMTP server
    certificate.

  

  
Optional lookup tables with the Postfix SMTP client TLS usage policy by
      next-hop destination and by remote SMTP server hostname.

  

  
Obsolete Postfix < 2.3 control for the Postfix SMTP client TLS cipher
      list.













  

  
The maximal number of parallel deliveries to the same destination via the
      smtp message delivery transport.

  

  
The maximal number of recipients per message for the smtp message delivery
      transport.

  

  
The Postfix SMTP client time limit for completing a TCP connection, or
      zero (use the operating system built-in time limit).

  

  
The Postfix SMTP client time limit for sending the HELO or EHLO command,
      and for receiving the initial remote SMTP server response.

  

  
The Postfix LMTP client time limit for sending the LHLO command, and for
      receiving the initial remote LMTP server response.

  

  
The Postfix SMTP client time limit for sending the XFORWARD command, and
      for receiving the remote SMTP server response.

  

  
The Postfix SMTP client time limit for sending the MAIL FROM command, and
      for receiving the remote SMTP server response.

  

  
The Postfix SMTP client time limit for sending the SMTP RCPT TO command,
      and for receiving the remote SMTP server response.

  

  
The Postfix SMTP client time limit for sending the SMTP DATA command, and
      for receiving the remote SMTP server response.

  

  
The Postfix SMTP client time limit for sending the SMTP message
    content.

  

  
The Postfix SMTP client time limit for sending the SMTP ".", and
      for receiving the remote SMTP server response.

  

  
The Postfix SMTP client time limit for sending the QUIT command, and for
      receiving the remote SMTP server response.




Available in Postfix version 2.1 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).

  

  
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).

  

  
The Postfix SMTP client time limit for sending the RSET command, and for
      receiving the remote SMTP server response.




Available in Postfix version 2.2 and earlier:



  

  
Keep Postfix LMTP client connections open for up to $max_idle
    seconds.




Available in Postfix version 2.2 and later:



  

  
Permanently enable SMTP connection caching for the specified
    destinations.

  

  
Temporarily enable SMTP connection caching while a destination has a high
      volume of mail in the active queue.

  

  
The amount of time during which Postfix will use an SMTP connection
      repeatedly.

  

  
When SMTP connection caching is enabled, the amount of time that an unused
      SMTP client socket is kept open before it is closed.




Available in Postfix version 2.3 and later:



  

  
Time limit for connection cache connect, send or receive operations.




Available in Postfix version 2.9 and later:



  

  
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).




Available in Postfix version 2.11 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).












Preliminary SMTPUTF8 support is introduced with Postfix 3.0.



  

  
Enable preliminary SMTPUTF8 support for the protocols described in RFC
      6531..6533.

  

  
Detect that a message requires SMTPUTF8 support for the specified mail
      origin classes.




Available in Postfix version 3.2 and later:



  

  
Enable 'transitional' compatibility between IDNA2003 and IDNA2008, when
      converting UTF-8 domain names to/from the ASCII form that is used for DNS
      lookups.













  

  
The increment in verbose logging level when a remote client or server
      matches a pattern in the debug_peer_list parameter.

  

  
Optional list of remote client or server hostname or network address
      patterns that cause the verbose logging level to increase by the amount
      specified in $debug_peer_level.

  

  
The recipient of postmaster notifications about mail delivery problems
      that are caused by policy, resource, software or protocol errors.

  

  
What categories of Postfix-generated mail are subject to before-queue
      content inspection by non_smtpd_milters, header_checks and
    body_checks.

  

  
The list of error classes that are reported to the postmaster.













  

  
Where the Postfix SMTP client should deliver mail when it detects a
      "mail loops back to myself" error condition.

  

  
The default location of the Postfix main.cf and master.cf configuration
      files.

  

  
How much time a Postfix daemon process may take to handle a request before
      it is terminated by a built-in watchdog timer.

  

  
The maximal number of digits after the decimal point when logging
      sub-second delay values.

  

  
Disable DNS lookups in the Postfix SMTP and LMTP clients.

  

  
The network interface addresses that this mail system receives mail
    on.

  

  
The Internet protocols Postfix will attempt to use when making or
      accepting connections.

  

  
The time limit for sending or receiving information over an internal
      communication channel.

  

  
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 TCP port that the Postfix LMTP client connects to.

  

  
The maximum amount of time that an idle Postfix daemon process waits for
      an incoming connection before terminating voluntarily.

  

  
The maximal number of incoming connections that a Postfix daemon process
      will service before terminating voluntarily.

  

  
The process ID of a Postfix command or daemon process.

  

  
The process name of a Postfix command or daemon process.

  

  
The network interface addresses that this mail system receives mail on by
      way of a proxy or network address translation unit.

  

  
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.

  

  
An optional numerical network address that the Postfix SMTP client should
      bind to when making an IPv4 connection.

  

  
An optional numerical network address that the Postfix SMTP client should
      bind to when making an IPv6 connection.

  

  
The hostname to send in the SMTP HELO or EHLO command.

  

  
The hostname to send in the LMTP LHLO command.

  

  
What mechanisms the Postfix SMTP client uses to look up a host's IP
      address.

  

  
Randomize the order of equal-preference MX host addresses.

  

  
The syslog facility of Postfix logging.

  

  
A prefix that is prepended to the process name in syslog records, so that,
      for example, "smtpd" becomes "prefix/smtpd".




Available with Postfix 2.2 and earlier:



  

  
Optional list of relay hosts for SMTP destinations that can't be found or
      that are unreachable.




Available with Postfix 2.3 and later:



  

  
Optional list of relay hosts for SMTP destinations that can't be found or
      that are unreachable.




Available with Postfix 3.0 and later:



  

  
In the context of email address verification, the SMTP protocol stage that
      determines whether an email address is deliverable.




Available with Postfix 3.1 and later:



  

  
Optional list of relay hosts for LMTP destinations that can't be found or
      that are unreachable.




Available with Postfix 3.2 and later:



  

  
The default TCP port that the Postfix SMTP client connects to.










generic(5), output address rewriting
header_checks(5), message header content inspection
body_checks(5), body parts content inspection
qmgr(8), queue manager
bounce(8), delivery status reports
scache(8), connection cache server
postconf(5), configuration parameters
master(5), generic daemon options
master(8), process manager
tlsmgr(8), TLS session and PRNG management
syslogd(8), system logging










Use "postconf readme_directory" or
    "postconf html_directory" to locate this information.


SASL_README, Postfix SASL howto
TLS_README, Postfix STARTTLS howto










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
Command pipelining in cooperation with:
Jon Ribbens
Oaktree Internet Solutions Ltd.,
Internet House,
Canal Basin,
Coventry,
CV1 4LY, United Kingdom.
SASL support originally by:
Till Franke
SuSE Rhein/Main AG
65760 Eschborn, Germany
TLS support originally by:
Lutz Jaenicke
BTU Cottbus
Allgemeine Elektrotechnik
Universitaetsplatz 3-4
D-03044 Cottbus, Germany
Revised TLS and SMTP connection cache support by:
Victor Duchovni
Morgan Stanley