July 12, 2006. Many, but not all, milters can be used with Postfix "out of the box". Interface The Sendmail Consortium provides a documented Application Programming Interface that can be used to create milters for special needs. The library provided by The Sendmail Consortium is pthread-based. However, documentation of the protocol used for communication between sendmail and milter processes is not provided. This internal protocol is subject to changes in new sendmail versions. Sample Milters MIMEDefang Amavis References Message transfer agents