smtp - Postfix SMTP client

smtp [generic Postfix daemon options]

The Postfix SMTP client 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 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 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 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 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. Session caching can be enabled permanently for specific destinations.

The SMTP client is moderately security-sensitive. It talks to SMTP servers and to DNS servers on the network. The SMTP 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 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)

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

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_mx_lookup_error (no)

Ignore DNS MX lookups that produce no response.

smtp_always_send_ehlo (yes)

Always send EHLO at the start of an SMTP session.

smtp_never_send_ehlo (no)

Never send EHLO at the start of an SMTP session.

smtp_cname_overrides_servername (yes)

Allow DNS CNAME records to override the servername that the Postfix SMTP client uses for logging, SASL password lookup, TLS policy decisions, or TLS certificate verification.

smtp_defer_if_no_mx_address_found (no)

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

smtp_line_length_limit (990)

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

smtp_pix_workaround_delay_time (10s)

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

smtp_pix_workaround_threshold_time (500s)

How long a message must be queued before the PIX firewall "<CR><LF>.<CR><LF>" bug workaround is turned on.

smtp_quote_rfc821_envelope (yes)

Quote addresses in SMTP MAIL FROM and RCPT TO commands as required by RFC 821.

smtp_skip_5xx_greeting (yes)

Skip SMTP servers that greet with a 5XX status code (go away, do not try again later).

smtp_skip_quit_response (yes)

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

Available in Postfix version 2.0 and earlier:

smtp_skip_4xx_greeting (yes)

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

Available in Postfix version 2.2 and later:

smtp_discard_ehlo_keyword_address_maps (empty)

Lookup tables, indexed by the remote SMTP server address, with case insensitive lists of EHLO keywords (pipelining, starttls, auth, etc.) that the SMTP client will ignore in the EHLO response from a remote SMTP server.

smtp_discard_ehlo_keywords (empty)

A case insensitive list of EHLO keywords (pipelining, starttls, auth, etc.) that the SMTP client will ignore in the EHLO response from a remote SMTP server.

smtp_generic_maps (empty)

Optional lookup tables that perform address rewriting in the SMTP client, typically to transform a locally valid address into a globally valid address when sending mail across the Internet.

Available in Postfix version 2.0 and later:

disable_mime_output_conversion (no)

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

mime_boundary_length_limit (2048)

The maximal length of MIME multipart boundary strings.

mime_nesting_limit (100)

The maximal recursion level that the MIME processor will handle.

Available in Postfix version 2.1 and later:

smtp_send_xforward_command (no)

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

smtp_sasl_auth_enable (no)

Enable SASL authentication in the Postfix SMTP client.

smtp_sasl_password_maps (empty)

Optional SMTP client lookup tables with one username:password entry per remote hostname or domain.

smtp_sasl_security_options (noplaintext, noanonymous)

What authentication mechanisms the Postfix SMTP client is allowed to use.

Available in Postfix version 2.2 and later:

smtp_sasl_mechanism_filter (empty)

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

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

smtp_use_tls (no)

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

smtp_enforce_tls (no)

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

smtp_sasl_tls_security_options ($smtp_sasl_security_options)

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

smtp_starttls_timeout (300s)

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

smtp_tls_CAfile (empty)

The file with the certificate of the certification authority (CA) that issued the Postfix SMTP client certificate.

smtp_tls_CApath (empty)

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

smtp_tls_cert_file (empty)

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

smtp_tls_cipherlist (empty)

Controls the Postfix SMTP client TLS cipher selection scheme.

smtp_tls_dcert_file (empty)

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

smtp_tls_dkey_file ($smtp_tls_dcert_file)

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

smtp_tls_enforce_peername (yes)

When TLS encryption is enforced, require that the remote SMTP server hostname matches the information in the remote SMTP server certificate.

smtp_tls_key_file ($smtp_tls_cert_file)

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

smtp_tls_loglevel (0)

Enable additional Postfix SMTP client logging of TLS activity.

smtp_tls_note_starttls_offer (no)

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

smtp_tls_per_site (empty)

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

smtp_tls_scert_verifydepth (5)

The verification depth for remote SMTP server certificates.

smtp_tls_session_cache_database (empty)

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

smtp_tls_session_cache_timeout (3600s)

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

tls_daemon_random_bytes (32)

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).

smtp_destination_concurrency_limit ($default_destination_concurrency_limit)

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

smtp_destination_recipient_limit ($default_destination_recipient_limit)

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

smtp_connect_timeout (30s)

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

smtp_helo_timeout (300s)

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

smtp_xforward_timeout (300s)

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

smtp_mail_timeout (300s)

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

smtp_rcpt_timeout (300s)

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

smtp_data_init_timeout (120s)

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

smtp_data_xfer_timeout (180s)

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

smtp_data_done_timeout (600s)

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

smtp_quit_timeout (300s)

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

Available in Postfix version 2.1 and later:

smtp_mx_address_limit (0)

The maximal number of MX (mail exchanger) IP addresses that can result from mail exchanger lookups, or zero (no limit).

smtp_mx_session_limit (2)

The maximal number of SMTP sessions per delivery request before giving up or delivering to a fall-back relay host, or zero (no limit).

smtp_rset_timeout (20s)

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

Available in Postfix version 2.2 and later:

smtp_connection_cache_destinations (empty)

Permanently enable SMTP connection caching for the specified destinations.

smtp_connection_cache_on_demand (yes)

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

smtp_connection_cache_reuse_limit (10)

When SMTP connection caching is enabled, the number of times that an SMTP session is reused before it is closed.

smtp_connection_cache_time_limit (2s)

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

debug_peer_level (2)

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

debug_peer_list (empty)

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.

error_notice_recipient (postmaster)

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

notify_classes (resource, software)

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

best_mx_transport (empty)

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

config_directory (see 'postconf -d' output)

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

daemon_timeout (18000s)

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

disable_dns_lookups (no)

Disable DNS lookups in the Postfix SMTP and LMTP clients.

fallback_relay (empty)

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

inet_interfaces (all)

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

inet_protocols (ipv4)

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

ipc_timeout (3600s)

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

max_idle (100s)

The maximum amount of time that an idle Postfix daemon process waits for the next service request before exiting.

max_use (100)

The maximal number of connection requests before a Postfix daemon process terminates.

process_id (read-only)

The process ID of a Postfix command or daemon process.

process_name (read-only)

The process name of a Postfix command or daemon process.

proxy_interfaces (empty)

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

smtp_bind_address (empty)

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

smtp_bind_address6 (empty)

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

smtp_helo_name ($myhostname)

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

smtp_host_lookup (dns)

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

smtp_randomize_addresses (yes)

Randomize the order of equal-preference MX host addresses.

syslog_facility (mail)

The syslog facility of Postfix logging.

syslog_name (postfix)

The mail system name that is prepended to the process name in syslog records, so that "smtpd" becomes, for example, "postfix/smtpd".

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

Command pipelining in cooperation with: Jon Ribbens Oaktree Internet Solutions Ltd., Internet House, Canal Basin, Coventry, CV1 4LY, United Kingdom.

Connection caching in cooperation with: Victor Duchovni Morgan Stanley

TLS support originally by: Lutz Jaenicke BTU Cottbus Allgemeine Elektrotechnik Universitaetsplatz 3-4 D-03044 Cottbus, Germany