milter

Milter, short for Mail Filter, is a cross‑MTA API that allows external programs to filter and modify email during the SMTP dialogue between a mail transfer agent (MTA) and a message. Originating with the Sendmail project, milters were designed to run as separate processes that the MTA can invoke as messages pass through the mail pipeline. Today, milters are supported by a wide range of MTAs, including Postfix and Exim, making them a common mechanism for content scanning and policy enforcement.

A milter sits between the MTA and the mail content, receiving events for different stages of a message’s delivery. Typical events include connection establishment, HELO, envelope sender and recipients, header parsing, data transmission, and end of message. Based on these events, the milter can examine and modify the message, and can request actions from the MTA such as accepting, rejecting, or deferring the message, as well as adding or changing headers, rewriting recipients, or quarantining the mail. The protocol usually uses UNIX domain sockets or TCP connections and relies on a library such as libmilter for communication.

Common uses of milters include spam filtering, antivirus scanning, DKIM and DMARC signing or verification, content policy enforcement, and routing decisions based on message characteristics. Popular milters include OpenDKIM, OpenDMARC, and clamav-milter, among others. By decoupling filtering logic from the MTA, milters provide modular, reusable policies that can be updated independently of the mail server software.

See also: mail transfer agent, spam filtering, DKIM, DMARC, antivirus filtering.