Postfix manual - smtp(8)

    SMTP(8)                                                                SMTP(8)

    NAME
           smtp, lmtp - Postfix SMTP+LMTP client

    SYNOPSIS
           smtp [generic Postfix daemon options] [flags=DORX]

           lmtp [generic Postfix daemon options] [flags=DORX]

    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 process name, smtp or
           lmtp, controls the protocol, and the names of the configuration parame-
           ters that will be used.

           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 server lookup strategy is different for SMTP and LMTP, as described
           in the sections "SMTP SERVER LOOKUP" and "LMTP SERVER LOOKUP".

           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 SERVER LOOKUP
           The  Postfix  SMTP  client  supports multiple destinations separated by
           comma or whitespace (Postfix 3.5 and later).  Each destination is tried
           in the specified order.

           SMTP destinations have the following form:

           domainname

           domainname:service
                  Look  up  the mail exchangers for the specified domain, and con-
                  nect to the specified service (default: smtp).  Optionally, mail
                  exchangers may be looked up with SRV queries instead of MX; this
                  requires that service is given in symbolic form.

           [hostname]

           [hostname]:service
                  Look up the address(es) for the specified host, and  connect  to
                  the specified service (default: smtp).

           [address]

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

    LMTP SERVER LOOKUP
           The  Postfix  LMTP  client  supports multiple destinations separated by
           comma or whitespace (Postfix 3.5 and later).  Each destination is tried
           in the specified order.

           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:domainname

           inet:domainname:service
                  Look up the LMTP servers for the specified  domain  and  service
                  (default:  lmtp).   This  form is supported when SRV lookups are
                  enabled, and requires that service is in symbolic form.

           inet:hostname

           inet:hostname:service
                  Look up the address(es) for the specified host, and  connect  to
                  the  specified  service  (default:  lmtp).  When SRV lookups are
                  enabled, use the form [hostname] to force address lookups.

           inet:[address]

           inet:[address]:service
                  Connect to the  specified  local  or  remote  host  and  service
                  (default:   lmtp).    An  IPv6  address  must  be  formatted  as
                  [ipv6:address].

    SINGLE-RECIPIENT DELIVERY
           By default, the Postfix SMTP+LMTP  client  delivers  mail  to  multiple
           recipients  per delivery request. This is undesirable when prepending a
           Delivered-to: or X-Original-To: message header. To prevent Postfix from
           sending multiple recipients per delivery request, specify

               transport_destination_recipient_limit = 1

           in  the  Postfix main.cf file, where transport is the name in the first
           column of the Postfix master.cf entry for this mail delivery service.

    COMMAND ATTRIBUTE SYNTAX
           flags=DORX (optional)
                  Optional message processing flags.

                  D      Prepend a "Delivered-To: recipient" message  header  with
                         the  envelope  recipient address. Note: for this to work,
                         the transport_destination_recipient_limit must be 1  (see
                         SINGLE-RECIPIENT DELIVERY above for details).

                         The  D  flag  also  enforces loop detection: if a message
                         already contains a Delivered-To:  header  with  the  same
                         recipient  address, then the message is returned as unde-
                         liverable. The address comparison is case insensitive.

                         This feature is available as of Postfix 3.5.

                  O      Prepend an "X-Original-To: recipient" message header with
                         the recipient address as given to Postfix. Note: for this
                         to work, the  transport_destination_recipient_limit  must
                         be 1 (see SINGLE-RECIPIENT DELIVERY above for details).

                         This feature is available as of Postfix 3.5.

                  R      Prepend a "Return-Path: <sender>" message header with the
                         envelope sender address.

                         This feature is available as of Postfix 3.5.

                  X      Indicates that the delivery is final. This  flag  affects
                         the  status  reported  in  "success" DSN (delivery status
                         notification) messages, and  changes  it  from  "relayed"
                         into "delivered".

                         This feature is available as of Postfix 3.5.

    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 2782 (SRV resource records)
           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)
           RFC 8689 (SMTP REQUIRETLS extension, TLS-Required header)

    DIAGNOSTICS
           Problems  and  transactions  are  logged  to syslogd(8) or postlogd(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 reuse for TLS (without  closing  the  SMTP  or
           LMTP connection) is not supported before Postfix 3.4.

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

    CONFIGURATION PARAMETERS
           Postfix versions 2.3 and later implement the SMTP and LMTP client  with
           the  same program, and choose the protocol and configuration parameters
           based on the process name, smtp or lmtp.

           Most smtp_xxx configuration parameters have an lmtp_xxx "mirror" param-
           eter  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.

    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 (998)
                  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 Postfix SMTP client
                  turns on the PIX firewall "<CR><LF>.<CR><LF>" bug workaround for
                  delivery through firewalls with "smtp fixup" mode turned on.

           smtp_pix_workarounds (disable_esmtp, delay_dotcrlf)
                  A  list  that  specifies  zero or more workarounds for CISCO PIX
                  firewall bugs.

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

           smtp_quote_rfc821_envelope (yes)
                  Quote  addresses  in  Postfix  SMTP client MAIL FROM and RCPT TO
                  commands as required by RFC 5321.

           smtp_reply_filter (empty)
                  A mechanism to transform replies from remote  SMTP  servers  one
                  line at a time.

           smtp_skip_5xx_greeting (yes)
                  Skip remote SMTP servers that greet with a 5XX status code.

           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
                  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:

           smtp_cname_overrides_servername (version dependent)
                  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:

           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 Postfix LMTP client will ignore in the LHLO
                  response from a remote LMTP server.

           lmtp_discard_lhlo_keywords (empty)
                  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:

           send_cyrus_sasl_authzid (no)
                  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  auth-
                  cid's password.

           Available in Postfix version 2.5 and later:

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

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

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

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

           Available in Postfix version 2.6 and later:

           tcp_windowsize (0)
                  An optional workaround for routers that break TCP  window  scal-
                  ing.

           Available in Postfix version 2.8 and later:

           smtp_dns_resolver_options (empty)
                  DNS Resolver options for the Postfix SMTP client.

           Available in Postfix version 2.9 - 3.6:

           smtp_per_record_deadline (no)
                  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  mes-
                  sage).

           Available in Postfix version 2.9 and later:

           smtp_send_dummy_mail_auth (no)
                  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:

           smtp_dns_support_level (empty)
                  Level of DNS support in the Postfix SMTP client.

           Available in Postfix version 3.0 and later:

           smtp_delivery_status_filter ($default_delivery_status_filter)
                  Optional filter for the smtp(8) delivery  agent  to  change  the
                  delivery status code or explanatory text of successful or unsuc-
                  cessful deliveries.

           smtp_dns_reply_filter (empty)
                  Optional filter for Postfix SMTP client DNS lookup results.

           Available in Postfix version 3.3 and later:

           smtp_balance_inet_protocols (yes)
                  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.

           Available in Postfix 3.5 and later:

           info_log_address_format (external)
                  The email address form that will be used  in  non-debug  logging
                  (info, warning, etc.).

           Available in Postfix 3.6 and later:

           dnssec_probe (ns:.)
                  The  DNS query type (default: "ns") and DNS query name (default:
                  ".") that Postfix may use to determine whether DNSSEC validation
                  is available.

           known_tcp_ports   (lmtp=24,   smtp=25,  smtps=submissions=465,  submis-
           sion=587)
                  Optional  setting  that  avoids lookups in the services(5) data-
                  base.

           Available in Postfix version 3.7 and later:

           smtp_per_request_deadline (no)
                  Change the behavior of the smtp_*_timeout time  limits,  from  a
                  time  limit  per  plaintext or TLS read or write call, to a com-
                  bined time limit for sending a complete  SMTP  request  and  for
                  receiving a complete SMTP response.

           smtp_min_data_rate (500)
                  The  minimum  plaintext  data  transfer rate in bytes/second for
                  DATA    requests,    when    deadlines    are    enabled    with
                  smtp_per_request_deadline.

           header_from_format (standard)
                  The format of the Postfix-generated From: header.

           Available in Postfix version 3.8 and later:

           use_srv_lookup (empty)
                  Enables  discovery  for  the  specified service(s) using DNS SRV
                  records.

           ignore_srv_lookup_error (no)
                  When SRV record lookup fails, fall back  to  MX  or  IP  address
                  lookup as if SRV record lookup was not enabled.

           allow_srv_lookup_fallback (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.

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

           smtp_sasl_security_options (noplaintext, noanonymous)
                  Postfix SMTP client SASL security options; as of Postfix 2.3 the
                  list of available features depends on the SASL client  implemen-
                  tation 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 the Postfix SMTP client
                  passes 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.

           Available in Postfix version 2.5 and later:

           smtp_sasl_auth_cache_name (empty)
                  An optional table to prevent repeated SASL authentication  fail-
                  ures  with  the  same  remote SMTP server hostname, username and
                  password.

           smtp_sasl_auth_cache_time (90d)
                  The maximal age of an smtp_sasl_auth_cache_name entry before  it
                  is removed.

           smtp_sasl_auth_soft_bounce (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.

           Available in Postfix version 2.9 and later:

           smtp_send_dummy_mail_auth (no)
                  Whether  or  not to append the "AUTH=<>" option to the MAIL FROM
                  command in SASL-authenticated SMTP sessions.

           Available in Postfix version 3.9 and later:

           smtp_sasl_password_result_delimiter (:)
                  The delimiter between username and password in  sasl_passwd_maps
                  lookup results.

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

           smtp_tls_security_level (Postfix >= 3.11: may; Postfix < 3.11: empty)
                  The default SMTP TLS security level for the Postfix SMTP client.

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

           smtp_tls_CApath (empty)
                  Directory  with  PEM format Certification 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
                  Postfix  SMTP  client cipher list at mandatory TLS security lev-
                  els.

           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 (see 'postconf -d' output)
                  TLS protocols that the Postfix SMTP client will use with  manda-
                  tory TLS encryption.

           smtp_tls_scert_verifydepth (9)
                  The verification depth for remote SMTP server certificates.

           smtp_tls_secure_cert_match (nexthop, dot-nexthop)
                  How  the  Postfix  SMTP  client  verifies the server certificate
                  peername 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)
                  How the Postfix SMTP  client  verifies  the  server  certificate
                  peername 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 (see 'postconf -d' output)
                  The OpenSSL cipherlist for "high" grade ciphers.

           tls_medium_cipherlist (see 'postconf -d' output)
                  The OpenSSL cipherlist for "medium" or higher grade ciphers.

           tls_null_cipherlist (eNULL:!aNULL)
                  The  OpenSSL  cipherlist  for  "NULL" grade ciphers that provide
                  authentication without encryption.

           Available in in Postfix version 2.3..3.7:

           tls_low_cipherlist (see 'postconf -d' output)
                  The OpenSSL cipherlist for "low" or higher grade ciphers.

           tls_export_cipherlist (see 'postconf -d' output)
                  The OpenSSL cipherlist for "export" or higher grade ciphers.

           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.

           Available in Postfix version 2.5 and later:

           smtp_tls_fingerprint_cert_match (empty)
                  List of acceptable remote SMTP server  certificate  fingerprints
                  for   the   "fingerprint"  TLS  security  level  (smtp_tls_secu-
                  rity_level = fingerprint).

           smtp_tls_fingerprint_digest (see 'postconf -d' output)
                  The message digest  algorithm  used  to  construct  remote  SMTP
                  server certificate fingerprints.

           Available in Postfix version 2.6 and later:

           smtp_tls_protocols (see 'postconf -d' output)
                  TLS  protocols that the Postfix SMTP client will use with oppor-
                  tunistic TLS encryption.

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

           smtp_tls_eccert_file (empty)
                  File  with the Postfix SMTP client ECDSA certificate in PEM for-
                  mat.

           smtp_tls_eckey_file ($smtp_tls_eccert_file)
                  File with the Postfix SMTP client ECDSA private key in PEM  for-
                  mat.

           Available in Postfix version 2.7 and later:

           smtp_tls_block_early_mail_reply (no)
                  Try  to  detect  a mail hijacking attack based on a TLS protocol
                  vulnerability (CVE-2009-3555), where an attacker prepends  mali-
                  cious  HELO,  MAIL, RCPT, DATA commands to a Postfix SMTP client
                  TLS session.

           Available in Postfix version 2.8 and later:

           tls_disable_workarounds (see 'postconf -d' output)
                  List or bit-mask of OpenSSL bug work-arounds to disable.

           Available in Postfix version 2.11-3.1:

           tls_dane_digest_agility (on)
                  Configure RFC7671 DANE TLSA digest algorithm agility.

           tls_dane_trust_anchor_digest_enable (yes)
                  Enable support for RFC 6698 (DANE TLSA) DNS records that contain
                  digests of trust-anchors with certificate usage "2".

           Available in Postfix version 2.11 and later:

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

           smtp_tls_force_insecure_host_tlsa_lookup (no)
                  Lookup the associated DANE TLSA RRset even when  a  hostname  is
                  not an alias and its address records lie in an unsigned zone.

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

           Available in Postfix version 3.0 and later:

           smtp_tls_wrappermode (no)
                  Request  that the Postfix SMTP client connects using the SUBMIS-
                  SIONS (formerly called SMTPS)  protocol  instead  of  using  the
                  STARTTLS command.

           Available in Postfix version 3.1 and later:

           smtp_tls_dane_insecure_mx_policy (dane)
                  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.

           Available in Postfix version 3.2 and later:

           tls_eecdh_auto_curves (see 'postconf -d' output)
                  The  prioritized list of elliptic curves, that should be enabled
                  in the Postfix SMTP client and server.

           Available in Postfix version 3.4 and later:

           smtp_tls_connection_reuse (no)
                  Try to make multiple deliveries per TLS-encrypted connection.

           smtp_tls_chain_files (empty)
                  List of one or more PEM files, each holding one or more  private
                  keys directly followed by a corresponding certificate chain.

           smtp_tls_servername (empty)
                  Optional  name  to  send  to  the  remote SMTP server in the TLS
                  Server Name Indication (SNI) extension.

           Available in Postfix 3.5, 3.4.6, 3.3.5, 3.2.10, 3.1.13 and later:

           tls_fast_shutdown_enable (yes)
                  A workaround for implementations that hang Postfix  while  shut-
                  ting down a TLS session, until Postfix times out.

           Available in Postfix version 3.8 and later:

           tls_ffdhe_auto_groups (see 'postconf -d' output)
                  The  prioritized  list  of finite-field Diffie-Hellman ephemeral
                  (FFDHE) key exchange groups supported by the Postfix SMTP client
                  and server.

           Available in Postfix 3.9, 3.8.1, 3.7.6, 3.6.10, 3.5.20 and later:

           tls_config_file (default)
                  Optional configuration file with baseline OpenSSL settings.

           tls_config_name (empty)
                  The  application  name passed by Postfix to OpenSSL library ini-
                  tialization functions.

           Available in Postfix version 3.9 and later:

           smtp_tls_enable_rpk (no)
                  Request that remote SMTP servers send an RFC7250 raw public  key
                  instead of an X.509 certificate.

           Available in Postfix version 3.10 and later:

           smtp_tlsrpt_enable (no)
                  Enable support for RFC 8460 TLSRPT notifications.

           smtp_tlsrpt_socket_name (empty)
                  The pathname of a UNIX-domain datagram socket that is managed by
                  a local TLSRPT reporting service.

           smtp_tlsrpt_skip_reused_handshakes (Postfix >= 3.11: no, Postfix  3.10:
           yes)
                  When set to "yes", report the TLSRPT status only for  "new"  TLS
                  sessions.

           tls_required_enable (yes)
                  Enable  support  for  the  "TLS-Required:  no"  message  header,
                  defined in RFC 8689.

           Available in Postfix version 3.10.5 and later:

           smtp_tls_enforce_sts_mx_patterns (yes)
                  Transform the TLS policy from an STS policy plugin:  connect  to
                  an  MX host only if its name matches any STS policy MX host pat-
                  tern, and match the server certificate against the MX  hostname.

           Available in Postfix version 3.11 and later:

           requiretls_enable (yes)
                  Enable  support  for  the  ESMTP  verb "REQUIRETLS" in the "MAIL
                  FROM" command.

           smtp_requiretls_policy (see 'postconf -d  smtp_requiretls_policy'  out-
           put)
                  How the Postfix SMTP and LMTP client will enforce REQUIRETLS for
                  messages received with the REQUIRETLS option.

           smtp_log_tls_feature_status (yes)
                  Enable  logging  of  TLS  feature information in delivery status
                  logging.

    OBSOLETE TLS 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_connect_timeout (30s)
                  The  Postfix SMTP client time limit for completing a TCP connec-
                  tion, or zero (use the operating system built-in time limit).

           smtp_helo_timeout (300s)
                  The Postfix SMTP client time limit for sending the HELO or  EHLO
                  command,  and  for  receiving  the  initial  remote  SMTP server
                  response.

           lmtp_lhlo_timeout (300s)
                  The Postfix LMTP client time limit for sending the LHLO command,
                  and for receiving the initial remote LMTP server response.

           smtp_xforward_timeout (300s)
                  The Postfix SMTP client time limit for sending the XFORWARD com-
                  mand, and for receiving the remote SMTP server response.

           smtp_mail_timeout (300s)
                  The Postfix SMTP client time limit for  sending  the  MAIL  FROM
                  command, and for receiving the remote SMTP server response.

           smtp_rcpt_timeout (300s)
                  The  Postfix SMTP client time limit for sending the SMTP RCPT TO
                  command, and for receiving the remote SMTP server response.

           smtp_data_init_timeout (120s)
                  The Postfix SMTP client time limit for  sending  the  SMTP  DATA
                  command, and for receiving the remote SMTP server response.

           smtp_data_xfer_timeout (180s)
                  The  Postfix SMTP client time limit for sending the SMTP message
                  content.

           smtp_data_done_timeout (600s)
                  The Postfix SMTP client time limit for sending the SMTP ".", and
                  for receiving the remote SMTP server response.

           smtp_quit_timeout (300s)
                  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:

           smtp_mx_address_limit (5)
                  The maximal number of MX (mail exchanger) IP addresses that  can
                  result  from Postfix SMTP client mail exchanger lookups, or zero
                  (no limit).

           smtp_mx_session_limit (2)
                  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).

           smtp_rset_timeout (20s)
                  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:

           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.

           Available in Postfix version 2.9 - 3.6:

           smtp_per_record_deadline (no)
                  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  mes-
                  sage).

           Available in Postfix version 2.11 and later:

           smtp_connection_reuse_count_limit (0)
                  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).

           Available in Postfix version 3.4 and later:

           smtp_tls_connection_reuse (no)
                  Try to make multiple deliveries per TLS-encrypted connection.

           Available in Postfix version 3.7 and later:

           smtp_per_request_deadline (no)
                  Change  the  behavior  of the smtp_*_timeout time limits, from a
                  time limit per plaintext or TLS read or write call,  to  a  com-
                  bined  time  limit  for  sending a complete SMTP request and for
                  receiving a complete SMTP response.

           smtp_min_data_rate (500)
                  The minimum plaintext data transfer  rate  in  bytes/second  for
                  DATA    requests,    when    deadlines    are    enabled    with
                  smtp_per_request_deadline.

           Implemented in the qmgr(8) daemon:

           transport_destination_concurrency_limit   ($default_destination_concur-
           rency_limit)
                  A transport-specific override for  the  default_destination_con-
                  currency_limit parameter value, where transport is the master.cf
                  name of the message delivery transport.

           transport_destination_recipient_limit     ($default_destination_recipi-
           ent_limit)
                  A transport-specific override for the default_destination_recip-
                  ient_limit  parameter  value,  where  transport is the master.cf
                  name of the message delivery transport.

    SMTPUTF8 CONTROLS
           Preliminary SMTPUTF8 support is introduced with Postfix 3.0.

           smtputf8_enable (yes)
                  Enable preliminary SMTPUTF8 support for the protocols  described
                  in RFC 6531, RFC 6532, and RFC 6533.

           smtputf8_autodetect_classes (sendmail, verify)
                  Detect  that  a message requires SMTPUTF8 support for the speci-
                  fied mail origin classes.

           Available in Postfix version 3.2 and later:

           enable_idna2003_compatibility (no)
                  Enable  'transitional'  compatibility   between   IDNA2003   and
                  IDNA2008,  when  converting UTF-8 domain names to/from the ASCII
                  form that is used for DNS lookups.

    TROUBLE SHOOTING CONTROLS
           debug_peer_level (2)
                  The increment in verbose logging level when a  nexthop  destina-
                  tion,  remote client or server name or network address matches a
                  pattern given with the debug_peer_list parameter.

           debug_peer_list (empty)
                  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.

           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 delay values.

           disable_dns_lookups (no)
                  Disable DNS lookups in the Postfix SMTP and LMTP clients.

           inet_interfaces (all)
                  The  local  network  interface  addresses  that this mail system
                  receives mail on.

           inet_protocols (see 'postconf -d' output)
                  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_assume_final (no)
                  When a remote LMTP server announces no DSN support, assume  that
                  the  server performs final delivery, and send "delivered" deliv-
                  ery status notifications instead of "relayed".

           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 an incoming connection before terminating voluntarily.

           max_use (100)
                  The maximal number of incoming connections that a Postfix daemon
                  process will service before terminating voluntarily.

           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  remote  network  interface  addresses that this mail system
                  receives mail on by way of a proxy or network  address  transla-
                  tion unit.

           smtp_address_preference (any)
                  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.

           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 HELO or EHLO command.

           lmtp_lhlo_name ($myhostname)
                  The hostname to send in the LMTP LHLO command.

           smtp_host_lookup (dns)
                  What mechanisms 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 (see 'postconf -d' output)
                  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:

           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 destinations that will be  used  when  an
                  SMTP  destination  is not found, or when delivery fails due to a
                  non-permanent error.

           Available with Postfix 3.0 and later:

           smtp_address_verify_target (rcpt)
                  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:

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

           Available with Postfix 3.2 and later:

           smtp_tcp_port (smtp)
                  The default TCP port that the Postfix SMTP client connects to.

           Available in Postfix 3.3 and later:

           service_name (read-only)
                  The master.cf service name of a Postfix daemon process.

           Available in Postfix 3.7 and later:

           smtp_bind_address_enforce (no)
                  Defer delivery when the Postfix SMTP  client  cannot  apply  the
                  smtp_bind_address or smtp_bind_address6 setting.

    SEE ALSO
           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
           postlogd(8), Postfix logging
           syslogd(8), system logging

    README FILES
           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

           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

                                                                           SMTP(8)