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.

Contents

• Architecture
  • Design Goals
  • Implementing Filtering Policies
  • MTA - Filter communication
• Technical Overview
  • Initialization
  • Control flow
  • Multithreading
  • Resource Management
  • Signal Handling
• API Documentation
  • Library Control Functions
  • Data Access Functions
  • Message Modification Functions
  • Callbacks
• Installation and Configuration
  • Compiling and Installing Your Filter
  • Configuring Sendmail
• A Sample Filter