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Postfix Documentation
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How Milter applications plug into Postfix

The Postfix Milter implementation uses two different lists of mail filters: one list of filters that are used for SMTP mail only, and one list of filters that are used for non-SMTP mail. The two lists have different capabilities, which is unfortunate. Avoiding this would require major restructuring of Postfix.

  • The SMTP-only filters handle mail that arrives via the Postfix smtpd(8) server. They are typically used to filter unwanted mail and to sign mail from authorized SMTP clients. You specify SMTP-only Milter applications with the smtpd_milters parameter as described in a later section. Mail that arrives via the Postfix smtpd(8) server is not filtered by the non-SMTP filters that are described next.

  • The non-SMTP filters handle mail that arrives via the Postfix sendmail(1) command-line or via the Postfix qmqpd(8) server. They are typically used to digitally sign mail only. Although non-SMTP filters can be used to filter unwanted mail, they have limitations compared to the SMTP-only filters. You specify non-SMTP Milter applications with the non_smtpd_milters parameter as described in a later section.

For those who are familiar with the Postfix architecture, the figure below shows how Milter applications plug into Postfix. Names followed by a number are Postfix commands or server programs, while unnumbered names inside shaded areas represent Postfix queues. To avoid clutter, the path for local submission is simplified (the OVERVIEW document has a more complete description).

SMTP-only
filters
non-SMTP
filters
^
|
|
v
^
|
|
|
|
|
|
v
Network -> smtpd(8)
\
Network -> qmqpd(8) -> cleanup(8) -> incoming
/
pickup(8)
:
Local -> sendmail(1)
Postfix Documentation
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