SMTPD(8)

NAME
smtpd - Postfix SMTP server

SYNOPSIS
smtpd [generic Postfix daemon options]

DESCRIPTION
The SMTP server accepts network connection requests and performs zero or more SMTP transactions per connection. Each received message is piped through the cleanup(8) daemon, and is placed into the incoming queue as one single queue file. For this mode of operation, the program expects to be run from the master(8) process manager. Alternatively, the SMTP server takes an established connection on standard input and deposits messages directly into the maildrop queue. In this so-called stand-alone mode, the SMTP server can accept mail even while the mail system is not running. The SMTP server implements a variety of policies for connection requests, and for parameters given to HELO, ETRN, MAIL FROM, VRFY and RCPT TO commands. They are detailed below and in the main.cf configuration file.

SECURITY
The SMTP server is moderately security-sensitive. It talks to SMTP clients and to DNS servers on the network. The SMTP server can be run chrooted at fixed low privilege.

STANDARDS
RFC 821 (SMTP protocol)
RFC 1123 (Host requirements)
RFC 1652 (8bit-MIME transport)
RFC 1869 (SMTP service extensions)
RFC 1870 (Message Size Declaration)
RFC 1985 (ETRN command)
RFC 2554 (AUTH command)
RFC 2821 (SMTP protocol)
RFC 2920 (SMTP Pipelining)

DIAGNOSTICS
Problems and transactions are logged to syslogd(8). Depending on the setting of the notify_classes parameter, the postmaster is notified of bounces, protocol problems, policy violations, and of other trouble.

CONFIGURATION PARAMETERS
The following main.cf parameters are especially relevant to this program. See the Postfix main.cf file for syntax details and for default values. Use the postfix reload command after a configuration change.

Compatibility controls

strict_rfc821_envelopes
    Disallow non-RFC 821 style addresses in SMTP commands. For example, the RFC822-style address forms with comments that Sendmail allows.

broken_sasl_auth_clients
    Support older Microsoft clients that mis-implement the AUTH protocol, and that expect an EHLO response of "250 AUTH=list" instead of "250 AUTH list".

smtpd_noop_commands
    List of commands that are treated as NOOP (no operation) commands, without any parameter syntax checking and without any state change. This list overrides built-in command definitions.

Content inspection controls
Optionally, Postfix can be configured to send new mail to external content filter software AFTER the mail is queued.

content_filter
    The name of a mail delivery transport that filters mail and that either bounces mail or re-injects the result back into Postfix. This parameter uses the same syntax as the right-hand side of a Postfix transport table.

receive_override_options
    The following options override main.cf settings. The options are either implemented by the SMTP server or are passed on to the downstream cleanup server.

    no_unknown_recipient_checks
        Do not try to reject unknown recipients. This is typically specified with the SMTP server after an external content filter.

    no_address_mappings
        Disable canonical address mapping, virtual alias map expansion, address masquerading, and automatic BCC recipients. This is typically specified with the SMTP server before an external content filter.

    no_header_body_checks
        Disable header/body_checks. This is typically specified with the SMTP server after an external content filter.

Pass-through proxy
Optionally, the Postfix SMTP server can be configured to forward all mail to a proxy server, for example a real-time content filter, BEFORE mail is queued.

smtpd_proxy_filter
    The host:port of the SMTP proxy server. The host or host: portion is optional.

smtpd_proxy_timeout
    Timeout for connecting to, sending to and receiving from the SMTP proxy server.

smtpd_proxy_ehlo
    The hostname to use when sending an EHLO command to the SMTP proxy server.

Authentication controls

smtpd_sasl_auth_enable
    Enable per-session authentication as per RFC 2554 (SASL). This functionality is available only when explicitly selected at program build time and explicitly enabled at runtime.

smtpd_sasl_local_domain
    The name of the local authentication realm.

smtpd_sasl_security_options
    Zero or more of the following.

    noplaintext
        Disallow authentication methods that use plaintext passwords.

    noactive
        Disallow authentication methods that are vulnerable to non-dictionary active attacks.

    nodictionary
        Disallow authentication methods that are vulnerable to passive dictionary attack.

    noanonymous
        Disallow anonymous logins.

smtpd_sender_login_maps
    Maps that specify the SASL login name that owns a MAIL FROM sender address. Used by the reject_sender_login_mismatch sender anti-spoofing restriction.

Miscellaneous

authorized_verp_clients
    Hostnames, domain names and/or addresses of clients that are authorized to use the XVERP extension.

debug_peer_level
    Increment in verbose logging level when a remote host matches a pattern in the debug_peer_list parameter.

debug_peer_list
    List of domain or network patterns. When a remote host matches a pattern, increase the verbose logging level by the amount specified in the debug_peer_level parameter.

default_verp_delimiters
    The default VERP delimiter characters that are used when the XVERP command is specified without explicit delimiters.

error_notice_recipient
    Recipient of protocol/policy/resource/software error notices.

hopcount_limit
    Limit the number of Received: message headers.

notify_classes
    List of error classes. Of special interest are:

    policy
        When a client violates any policy, mail a transcript of the entire SMTP session to the postmaster.

    protocol
        When a client violates the SMTP protocol or issues an unimplemented command, mail a transcript of the entire SMTP session to the postmaster.

smtpd_banner
    Text that follows the 220 status code in the SMTP greeting banner.

smtpd_expansion_filter
    Controls what characters are allowed in $name expansion of rbl template responses and other text.

smtpd_recipient_limit
    Restrict the number of recipients that the SMTP server accepts per message delivery.

smtpd_timeout
    Limit the time to send a server response and to receive a client request.

soft_bounce
    Change hard (5xx) reject responses into soft (4xx) reject responses. This can be useful for testing purposes.

verp_delimiter_filter
    The characters that Postfix accepts as VERP delimiter characters.

Known versus unknown recipients

show_user_unknown_table_name
    Whether or not to reveal the table name in the "User unknown" responses. The extra detail makes trouble shooting easier but also reveals information that is nobody elses business.

unknown_local_recipient_reject_code
    The response code when a client specifies a recipient whose domain matches $mydestination or $inet_interfaces, while $local_recipient_maps is non-empty and does not list the recipient address or address local-part.

unknown_relay_recipient_reject_code
    The response code when a client specifies a recipient whose domain matches $relay_domains, while $relay_recipient_maps is non-empty and does not list the recipient address.

unknown_virtual_alias_reject_code
    The response code when a client specifies a recipient whose domain matches $virtual_alias_domains, while the recipient is not listed in $virtual_alias_maps.

unknown_virtual_mailbox_reject_code
    The response code when a client specifies a recipient whose domain matches $virtual_mailbox_domains, while the recipient is not listed in $virtual_mailbox_maps.

Resource controls

line_length_limit
    Limit the amount of memory in bytes used for the handling of partial input lines.

message_size_limit
    Limit the total size in bytes of a message, including on-disk storage for envelope information.

queue_minfree
    Minimal amount of free space in bytes in the queue file system for the SMTP server to accept any mail at all.

smtpd_history_flush_threshold
    Flush the command history to postmaster after receipt of RSET etc. only if the number of history lines exceeds the given threshold.

Tarpitting

smtpd_error_sleep_time
    Time to wait in seconds before sending a 4xx or 5xx server error response.

smtpd_soft_error_limit
    When an SMTP client has made this number of errors, wait error_count seconds before responding to any client request.

smtpd_hard_error_limit
    Disconnect after a client has made this number of errors.

smtpd_junk_command_limit
    Limit the number of times a client can issue a junk command such as NOOP, VRFY, ETRN or RSET in one SMTP session before it is penalized with tarpit delays.

Delegated policy

smtpd_policy_service_timeout
    Time limit for connecting to, writing to and receiving from a delegated SMTPD policy server.

smtpd_policy_service_max_idle
    Time after which an unused SMTPD policy service connection is closed.

smtpd_policy_service_timeout
    Time after which an active SMTPD policy service connection is closed.

UCE control restrictions

parent_domain_matches_subdomains
    List of Postfix features that use domain.tld patterns to match sub.domain.tld (as opposed to requiring .domain.tld patterns).

smtpd_client_restrictions
    Restrict what clients may connect to this mail system.

smtpd_helo_required
    Require that clients introduce themselves at the beginning of an SMTP session.

smtpd_helo_restrictions
    Restrict what client hostnames are allowed in HELO and EHLO commands.

smtpd_sender_restrictions
    Restrict what sender addresses are allowed in MAIL FROM commands.

smtpd_recipient_restrictions
    Restrict what recipient addresses are allowed in RCPT TO commands.

smtpd_etrn_restrictions
    Restrict what domain names can be used in ETRN commands, and what clients may issue ETRN commands.

smtpd_data_restrictions
    Restrictions on the DATA command. Currently, the only restriction that makes sense here is reject_unauth_pipelining.

allow_untrusted_routing
    Allow untrusted clients to specify addresses with sender-specified routing. Enabling this opens up nasty relay loopholes involving trusted backup MX hosts.

smtpd_restriction_classes
    Declares the name of zero or more parameters that contain a list of UCE restrictions. The names of these parameters can then be used instead of the restriction lists that they represent.

smtpd_null_access_lookup_key
    The lookup key to be used in SMTPD access tables instead of the null sender address. A null sender address cannot be looked up.

maps_rbl_domains (deprecated)
    List of DNS domains that publish the addresses of blacklisted hosts. This is used with the deprecated reject_maps_rbl restriction.

permit_mx_backup_networks
    Only domains whose primary MX hosts match the listed networks are eligible for the permit_mx_backup feature.

relay_domains
    Restrict what domains this mail system will relay mail to. The domains are routed to the delivery agent specified with the relay_transport setting.

Sender/recipient address verification
Address verification is implemented by sending probe email messages that are not actually delivered, and is enabled via the reject_unverified_{sender,recipient} access restriction. The status of verification probes is maintained by the address verification service.

address_verify_poll_count
    How many times to query the address verification service for completion of an address verification request. Specify 1 to implement a simple form of greylisting, that is, always defer the request for a new sender or recipient address.

address_verify_poll_delay
    Time to wait after querying the address verification service for completion of an address verification request.

UCE control responses

access_map_reject_code
    Response code when a client violates an access database restriction.

default_rbl_reply
    Default template reply when a request is RBL blacklisted. This template is used by the reject_rbl_* and reject_rhsbl_* restrictions. See also: rbl_reply_maps and smtpd_expansion_filter.

defer_code
    Response code when a client request is rejected by the defer restriction.

invalid_hostname_reject_code
    Response code when a client violates the reject_invalid_hostname restriction.

maps_rbl_reject_code
    Response code when a request is RBL blacklisted.

multi_recipient_bounce_reject_code
    Response code when a multi-recipient bounce is blocked.

rbl_reply_maps
    Table with template responses for RBL blacklisted requests, indexed by RBL domain name. These templates are used by the reject_rbl_* and reject_rhsbl_* restrictions. See also: default_rbl_reply and smtpd_expansion_filter.

reject_code
    Response code when the client matches a reject restriction.

relay_domains_reject_code
    Response code when a client attempts to violate the mail relay policy.

unknown_address_reject_code
    Response code when a client violates the reject_unknown_address restriction.

unknown_client_reject_code
    Response code when a client without address to name mapping violates the reject_unknown_client restriction.

unknown_hostname_reject_code
    Response code when a client violates the reject_unknown_hostname restriction.

unverified_sender_reject_code
    Response code when a sender address is known to be undeliverable.

unverified_recipient_reject_code
    Response code when a recipient address is known to be undeliverable.

SEE ALSO
cleanup(8) message canonicalization
master(8) process manager
syslogd(8) system logging
trivial-rewrite(8) address resolver
verify(8) address verification service

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