A milter is an extension to the widely used open source mail transfer agent (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

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Sendmail

Milter (which is short for Mail Filter) was first introduced to Sendmail 8.12.0 released in September 8 2001 and has undergone impressive improvements since then.

Postfix

Milter support was added to Postfix ^* 2.3.0 released July 122006

Interface

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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 for protocol used for communnication between sendmail and milter processes.

Milters

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• MIMEDefang

External links

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• www.milter.org Milter.org
• www.milter.info Milter.info
• Sendmail::Milter
• Milter protocol documentation (unofficial) - Milter API is officially documented
• MIMEDefang Milter (home site)