SMTP Client Options

The SMTP client options configures how Postfix will behave when dealing with other mail servers as a client, e.g., when sending mail on behalf of a user. This portion of the configuration primarily dictates how the server will respond to certain error conditions.

Action when listed as best MX server

As discussed in the BIND chapter, a mail server performs a name server query to find the MX, or mail server, record for the destination domain. If this record indicates that the local server is the server to which mail should be sent, it can respond in a couple of ways. The default is to bounce the message with an error indicating a mail loop. If the field is selected and local is entered, the mail will be directed to the local delivery agent instead of bouncing the mail. This option correlates to the best_mx_transport directive.

Hosts/domains to hand off mail to on invalid destination

By default, mail that cannot be delivered because the destination is invalid will be bounced with an appropriate error message. However, it is possible to configure postfix to hand off email to another server instead. This option correlates to the fallback_relay directive.

Ignore MX lookup error

If a name server query fails to provide an MX record, the server defaults to deferring the mail and trying again later. If Yes is selected instead, an A record query will be done and an attempt will be made to deliver to the resulting address. This option correlates to the ignore_mx_lookup_error directive.

Skip 4xx greeting

If a remote server responds to a connection with a 4XX status code, Postfix will, by default, select the next available mail exchanger specified by the MX records. If set to No, mail delivery will be deferred after the first mail delivery attempt, and another attempt will be made later. This option correlates to the smtp_skip_4xx_greeting directive.

Skip wait for the QUIT command

This option configures whether Postfix will wait for the receiving mail server to respond to the QUIT command. This option correlates to the smtp_skip_quit_response directive and defaults to no.

Max number of parallel deliveries to the same destination

This option specifies the maximum number of deliveries that Postfix will perform to the same destination simultaneously. This option correlates to the smtp_destination_concurrency_limit directive and defaults to the system-wide limit for parallel deliveries configured in the Delivery Rates page documented in the next section.

Max number of recipients per delivery

Limits the number of recipients per delivery. This option correlates to the smtp_destination_recipient_limit directive and defaults to the system-wide limit for recipients per delivery.

Timeout for completing TCP connections

Specifies the time in seconds that the Postfix delivery agent will wait before timing out a TCP connection. This option correlates to the smtp_connect_timeout directive and defaults to 0, which disables connection timeouts.

Timeout on waiting for the greeting banner

Limits how long Postfix will wait for a greeting banner to be received from the destination server. This option corresponds to the smtp_helo_timeout directive and defaults to 300 seconds.

Timeout on waiting for answer to MAIL FROM

Sets the timeout in seconds for sending the SMTP MAIL FROM command and for receiving the destination server’s response. This option correlates to the smtp_mail_timeout directive and defaults to 300 seconds.

Timeout on waiting for answer to RCPT TO

Sets the timeout in seconds for sending the SMTP RCPT TO command and for receiving the destination server’s response. This option correlates to the smtp_rcpt_timeout directive and defaults to 300 seconds.

Timeout on waiting for answer to DATA

Sets the timeout in seconds sending the SMTP DATA command and for receiving the destination server’s response. This option correlates to the smtp_data_init_timeout directive and defaults to 120 seconds.

Timeout on waiting for answer to transmit of message content

Specifies the SMTP client timeout in seconds for sending the contents of the message. If the connection stalls for longer than this timeout, the delivery agent will terminate to transfer. This option corresponds to the smtp_data_xfer_timeout directive and defaults to 180 seconds.

Timeout on waiting for answer to ending “.”

Specifies the SMTP client timeout in seconds for sending the closing SMTP “.” and receiving the destination server’s reply. This option correlates to the smtp_data_done_timeout directive and defaults to 600 seconds.

Timeout on waiting for answer to QUIT

Sets the timeout in seconds sending the SMTP QUIT command and for receiving the destination server’s response. This option correlates to the smtp_quit_timeout directive and defaults to 300 seconds.