A milter (portmanteau for mail filter) is an extension to the widely used open source mail transfer agents (MTA) Sendmail and Postfix. It allows administrators to add mail filters (for filtering spam, viruses ...) very efficiently in the mail-processing-chain of sendmail.

Milter filters can reject a message during the SMTP session. Milters preserve reliable email delivery without taking over responsibility for generating bounce messages (envelope sender addresses of viruses and spam are frequently faked).

Sendmail.org includes a special thread based library in the sendmail distribution which provides the milter API. The Sendmail daemon communicates with a milter process using the milter protocol.

The usage of milters has spread in recent years and milters are becoming increasingly important in mail processing on the large scale to block, verify and sort incoming mail.

MTA

Sendmail

Milter was first officially introduced to Sendmail 8.12.0 released in September 8, 2001. Milter was available in sendmail 8.11.x versions as an FFR (For Future Release/Beta) code.

Milter has undergone impressive improvements since its first official release.

Search for MILTER in SENDMAIL RELEASE NOTES to know about changes related to Milter in sendmail versions ^[1].

Milter will be someway also available in sendmail 8 successor's : Meta1 (previously labeled sendmail X)

Postfix

Milter support was added to Postfix 2.3.0 released July 12, 2006 ^[2].
Not all Milters can be used with Postfix out of the box, but it is a good news for Milter developers.

Postoffice SMTP server

Milter support since release 1.3

Interface

Sendmail provides a documented Application Programming Interface that can be used to create milters for special needs. The library provided by sendmail.org is thread based.

Sendmail.org does not provide documentation of the protocol used for communication between sendmail and milter processes. This internal protocol is subject to changes in new sendmail versions.

Sample Milters

• MIMEDefang
• j-chkmail

External links

• Milter API documentation
• Milter README
• www.milter.org Milter.org
• www.milter.info Milter.info
• Postoffice SMTP server

References

1. SENDMAIL RELEASE NOTES
2. Postfix before-queue Milter support

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Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Milter". Image Credit.

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