Milters

Milter, short for mail filter, is a standardized interface that allows external programs to participate in the handling of email messages as they pass through a mail transfer agent (MTA). Originating with Sendmail, milters are loaded by the MTA and communicate with it over a socket using the Milter protocol. This arrangement enables third-party software to inspect, modify, or make policy decisions about messages during the SMTP dialogue, often with low latency and without requiring the MTA to implement the filtering logic itself.

Operation: During a message's journey, the MTA invokes the milter at defined stages (connect, HELO, MAIL FROM, RCPT TO, headers, body, end of message). The milter can add or modify headers, rewrite addresses, reject, defer, or accept the message, and even modify the message body. The exact actions are defined by the protocol and by the MTA's configuration.

Common uses include anti-spam and antivirus filtering, DKIM and DMARC signing or policy evaluation, and content-based filtering. Milters are implemented as separate programs or libraries; many are open source. Popular milters include OpenDKIM for DKIM signing, OpenDMARC for DMARC policy checks, and clamav-milter for virus scanning; SpamAssassin can operate as a milter via spamass-milter.

Deployment notes: Milters run as separate processes or services and communicate over Unix domain sockets or TCP, often with libmilter. They require compatibility with the MTA and careful configuration to balance security and performance; misconfiguration can cause mail delays or bypass filters.