SMTP(8)SMTP(8)NAME
smtp - Postfix SMTP+LMTP client
SYNOPSIS
smtp [generic Postfix daemon options]
DESCRIPTION
The Postfix SMTP+LMTP client implements the SMTP and LMTP mail delivery
protocols. It processes message delivery requests from the queue man‐
ager. 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. Session caching
can be enabled permanently for specific destinations.
SMTP DESTINATION SYNTAX
SMTP destinations have the following form:
domainname
domainname:port
Look up the mail exchangers for the specified domain, and con‐
nect 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 format‐
ted as [ipv6:address].
LMTP DESTINATION SYNTAX
LMTP destinations have the following form:
unix:pathname
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.
inet:hostname
inet:hostname:port
inet:[address]
inet:[address]:port
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].
SECURITY
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.
STANDARDS
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)
DIAGNOSTICS
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 postmas‐
ter is notified of bounces, protocol problems, and of other trouble.
BUGS
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.
CONFIGURATION PARAMETERS
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 "ghost" parame‐
ter for the equivalent LMTP feature. This document describes only those
LMTP-related parameters that aren't simply "ghost" 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.
COMPATIBILITY CONTROLS
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_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 for delivery
through firewalls with "smtp fixup" mode 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 Postfix 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 Postfix 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 2.2.9 and later:
smtp_cname_overrides_servername (version dependent)
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.
Available in Postfix version 2.3 and later:
lmtp_discard_lhlo_keyword_address_maps (empty)
Lookup tables, indexed by the remote LMTP server address, with
case insensitive lists of LHLO keywords (pipelining, starttls,
auth, etc.) that the LMTP client will ignore in the LHLO
response from a remote LMTP server.
lmtp_discard_lhlo_keywords ($myhostname)
A case insensitive list of LHLO keywords (pipelining, starttls,
auth, etc.) that the LMTP client will ignore in the LHLO
response from a remote LMTP server.
MIME PROCESSING CONTROLS
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.
EXTERNAL CONTENT INSPECTION CONTROLS
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.
SASL AUTHENTICATION CONTROLS
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, or sender address when
sender-dependent authentication is enabled.
smtp_sasl_security_options (noplaintext, noanonymous)
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:
smtp_sasl_mechanism_filter (empty)
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:
smtp_sender_dependent_authentication (no)
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 dif‐
ferent senders will use the appropriate credentials.
smtp_sasl_path (empty)
Implementation-specific information that is passed through to
the SASL plug-in implementation that is selected with
smtp_sasl_type.
smtp_sasl_type (cyrus)
The SASL plug-in type that the Postfix SMTP client should use
for authentication.
STARTTLS SUPPORT CONTROLS
Detailed information about STARTTLS configuration may be found in the
TLS_README document.
smtp_tls_security_level (empty)
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.
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_mandatory_ciphers (medium)
The minimum TLS cipher grade that the Postfix SMTP client will
use with mandatory TLS encryption.
smtp_tls_exclude_ciphers (empty)
List of ciphers or cipher types to exclude from the Postfix SMTP
client cipher list at all TLS security levels.
smtp_tls_mandatory_exclude_ciphers (empty)
Additional list of ciphers or cipher types to exclude from the
SMTP client cipher list at mandatory TLS security levels.
smtp_tls_dcert_file (empty)
File with the Postfix SMTP client DSA certificate in PEM format.
smtp_tls_dkey_file ($smtp_tls_dcert_file)
File with the Postfix SMTP client DSA private key in PEM format.
smtp_tls_key_file ($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_policy_maps (empty)
Optional lookup tables with the Postfix SMTP client TLS security
policy by next-hop destination; when a non-empty value is speci‐
fied, this overrides the obsolete smtp_tls_per_site parameter.
smtp_tls_mandatory_protocols (SSLv3, TLSv1)
List of TLS protocols that the Postfix SMTP client will use with
mandatory TLS encryption.
smtp_tls_scert_verifydepth (5)
The verification depth for remote SMTP server certificates.
smtp_tls_secure_cert_match (nexthop, dot-nexthop)
The server certificate peername verification method for the
"secure" TLS security level.
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.
smtp_tls_verify_cert_match (hostname)
The server certificate peername verification method for the
"verify" TLS security level.
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).
tls_high_cipherlist (!EXPORT:!LOW:!MEDIUM:ALL:+RC4:@STRENGTH)
The OpenSSL cipherlist for "HIGH" grade ciphers.
tls_medium_cipherlist (!EXPORT:!LOW:ALL:+RC4:@STRENGTH)
The OpenSSL cipherlist for "MEDIUM" or higher grade ciphers.
tls_low_cipherlist (!EXPORT:ALL:+RC4:@STRENGTH)
The OpenSSL cipherlist for "LOW" or higher grade ciphers.
tls_export_cipherlist (ALL:+RC4:@STRENGTH)
The OpenSSL cipherlist for "EXPORT" or higher grade ciphers.
tls_null_cipherlist (!aNULL:eNULL+kRSA)
The OpenSSL cipherlist for "NULL" grade ciphers that provide
authentication without encryption.
Available in Postfix version 2.4 and later:
smtp_sasl_tls_verified_security_options ($smtp_sasl_tls_secu‐
rity_options)
The SASL authentication security options that the Postfix SMTP
client uses for TLS encrypted SMTP sessions with a verified
server certificate.
OBSOLETE STARTTLS CONTROLS
The following configuration parameters exist for compatibility with
Postfix versions before 2.3. Support for these will be removed in a
future release.
smtp_use_tls (no)
Opportunistic mode: use TLS when a remote SMTP server announces
STARTTLS support, otherwise send the mail in the clear.
smtp_enforce_tls (no)
Enforcement mode: require that remote SMTP servers use TLS
encryption, and never send mail in the clear.
smtp_tls_enforce_peername (yes)
With mandatory TLS encryption, require that the remote SMTP
server hostname matches the information in the remote SMTP
server certificate.
smtp_tls_per_site (empty)
Optional lookup tables with the Postfix SMTP client TLS usage
policy by next-hop destination and by remote SMTP server host‐
name.
smtp_tls_cipherlist (empty)
Obsolete Postfix < 2.3 control for the Postfix SMTP client TLS
cipher list.
RESOURCE AND RATE CONTROLS
smtp_destination_concurrency_limit ($default_destination_concur‐
rency_limit)
The maximal number of parallel deliveries to the same destina‐
tion via the smtp message delivery transport.
smtp_destination_recipient_limit ($default_destination_recipient_limit)
The maximal number of recipients per delivery via the smtp mes‐
sage 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.
lmtp_lhlo_timeout (300s)
The LMTP client time limit for sending the LHLO 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 (5)
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 earlier:
lmtp_cache_connection (yes)
Keep Postfix LMTP client connections open for up to $max_idle
seconds.
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_reuse_time_limit (300s)
The amount of time during which Postfix will use an SMTP connec‐
tion repeatedly.
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.
Available in Postfix version 2.3 and later:
connection_cache_protocol_timeout (5s)
Time limit for connection cache connect, send or receive opera‐
tions.
TROUBLE SHOOTING CONTROLS
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 proto‐
col errors.
internal_mail_filter_classes (empty)
What categories of Postfix-generated mail are subject to before-
queue content inspection by non_smtpd_milters, header_checks and
body_checks.
notify_classes (resource, software)
The list of error classes that are reported to the postmaster.
MISCELLANEOUS CONTROLS
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 con‐
figuration 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.
delay_logging_resolution_limit (2)
The maximal number of digits after the decimal point when log‐
ging sub-second delay values.
disable_dns_lookups (no)
Disable DNS lookups in the Postfix SMTP and LMTP clients.
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.
lmtp_tcp_port (24)
The default TCP port that the Postfix LMTP client connects to.
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 dae‐
mon 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 Postfix SMTP
client should bind to when making an IPv4 connection.
smtp_bind_address6 (empty)
An optional numerical network address that the Postfix 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.
lmtp_lhlo_name ($myhostname)
The hostname to send in the LMTP LHLO command.
smtp_host_lookup (dns)
What mechanisms when the Postfix SMTP client uses to look up a
host's IP address.
smtp_randomize_addresses (yes)
Randomize the order of equal-preference MX host addresses.
syslog_facility (mail)
The syslog facility of Postfix logging.
syslog_name (postfix)
The mail system name that is prepended to the process name in
syslog records, so that "smtpd" becomes, for example, "post‐
fix/smtpd".
Available with Postfix 2.2 and earlier:
fallback_relay (empty)
Optional list of relay hosts for SMTP destinations that can't be
found or that are unreachable.
Available with Postfix 2.3 and later:
smtp_fallback_relay ($fallback_relay)
Optional list of relay hosts for SMTP destinations that can't be
found or that are unreachable.
SEE ALSOqmgr(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
README FILES
Use "postconf readme_directory" or "postconf html_directory" to locate
this information.
SASL_README, Postfix SASL howto
TLS_README, Postfix STARTTLS howto
LICENSE
The Secure Mailer license must be distributed with this software.
AUTHOR(S)
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.
SASL support originally by:
Till Franke
SuSE Rhein/Main AG
65760 Eschborn, Germany
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
SMTP(8)
