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Sendmail's Content Management API (milter) provides third-party programs to access mail messages as they are being processed by the Mail Transfer Agent (MTA), allowing them to examine and modify message content and meta-information. Filtering policies implemented by Milter-conformant filters may then be centrally configured and composed in an end-user's MTA configuration file.

Possible uses for filters include spam rejection, virus filtering, and content control. In general, Milter seeks to address site-wide filtering concerns in a scalable way. Individual users' mail filtering needs (e.g. sorting messages by subject) are left to client-level programs such as Procmail.

This document is a technical introduction intended for those interested in developing Milter filters. It includes: