libmilters

libmilter is a C library that provides an implementation of the Milter (mail filter) protocol, enabling mail transfer agents (MTAs) to communicate with external programs called milters. Milters can inspect and modify SMTP messages in transit, enforce policies, apply antivirus and antispam checks, and reject or modify messages based on content, headers, or envelope data.

Developers write milters by linking against libmilter and supplying a descriptor with a name and version and a set of callback functions to handle events such as connection establishment, HELO, envelope from, envelope to, data, header parsing, and end-of-message. libmilter handles the underlying protocol negotiation, framing, and error reporting, and provides helpers to accept, defer, or reject messages according to the MTA’s decisions.

Usage: A milter program registers with the MTA via libmilter, then starts the event loop. The library translates MTA actions into callback results and communicates back to the MTA. Milters commonly implement content filtering (antivirus, antispam), policy checks, or header rewriting.

History and scope: libmilter originated within the Sendmail project and has been adopted by multiple MTAs and filtering programs as a standard interface for external filters. It remains a core component for developers building external filter programs that integrate with MTAs through the Milter protocol.

See also: Milter protocol, milters, clamav-milter, spamass-milter.