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Postfix Documentation
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Limitations

This section lists limitations of the Postfix Milter implementation. Some limitations will be removed as the implementation is extended over time. Of course the usual limitations of before-queue filtering will always apply. See the CONTENT_INSPECTION_README document for a discussion.

  • Postfix currently supports only applications that speak the Sendmail 8 Milter protocol versions 2..4. Support for other protocol types or protocol versions may be added later.

  • For applications that are written in C, you need to use the Sendmail libmilter library. A Postfix replacement may be provided in the future.

  • There are TWO sets of mail filters: filters that are used for SMTP mail only (specified with the smtpd_milters parameter), and filters for non-SMTP mail (specified with the non_smtpd_milters parameter). The non-SMTP filters are primarily for local submissions.

  • When mail is filtered by non-SMTP filters, the Postfix cleanup(8) server has to simulate the SMTP client CONNECT and DISCONNECT events, and the SMTP client EHLO, MAIL FROM, RCPT TO and DATA commands. This works as expected, with only one exception: non-SMTP filters must not REJECT or TEMPFAIL simulated RCPT TO commands. When a non-SMTP filter REJECTs or TEMPFAILs a recipient, Postfix will report a configuration error, and mail will stay in the queue.

  • Postfix currently does not apply content filters to mail that is forwarded or aliased internally, or to mail that is generated internally such as bounces or Postmaster notifications. This may be a problem when you want to apply a signing Milter to such mail.

  • When you use the before-queue content filter for incoming SMTP mail (see SMTPD_PROXY_README), Milter applications have access only to the SMTP command information; they have no access to the message header or body, and cannot make modifications to the message or to the envelope.

  • Postfix 2.3 does not support Milter requests to replace the message body. Milter applications that request this unsupported operation will log a warning like this:

    application name: st_optionneg[134563840]: 0x3d does not fulfill action requirements 0x1e
    

    The solution is (to wait for) a Postfix version that supports the missing functionality.

  • Most Milter configuration options are global. Future Postfix versions may support per-Milter timeouts, per-Milter error handling, etc.

Postfix Documentation
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