Filtering Mail with Sendmail
Introduction

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:

 A description of Milter's design goals.
 An explanation of Milter application architecture, including interactions
between the support library and user code, and between filters and the MTA.
 A specification of the C application programming interface.
 An example of a simple Milter filter.