Postfix before-queue Milter support

Introduction

Postfix version 2.3 introduces support for the Sendmail version 8 Milter (mail filter) protocol. This protocol is used by applications that run outside the MTA to inspect SMTP events (CONNECT, DISCONNECT), SMTP commands (HELO, MAIL FROM, etc.) as well as mail content. All this happens before mail is queued.

The reason for adding Milter support to Postfix is that there exists a large collection of applications, not only to block unwanted mail, but also to verify authenticity (examples: SenderID+SPF and Domain keys) or to digitally sign mail (example: Domain keys). Having yet another Postfix-specific version of all that software is a poor use of human and system resources.

Postfix 2.3 implements all the requests of Sendmail version 8 Milter protocols up to version 4, except one: message body replacement. See, however, the workarounds and limitations sections at the end of this document.

This document provides information on the following topics:

• How Milter applications plug into Postfix
• Building Milter applications
• Running Milter applications
• Configuring Postfix
• Workarounds
• Limitations