The Postfix before-queue content filter feature

As of version 2.1, the Postfix SMTP server can forward all incoming mail to a content filtering proxy server that inspects all mail BEFORE it is stored in the Postfix mail queue.

The before-queue content filter is meant to be used as follows:

Internet

->

Postfix SMTP server

->

Before queue filter

->

Postfix SMTP server

->

Postfix cleanup server

->

Postfix queue

-<

smtp local virtual

The before-queue content filter is not to be confused with the approach described in the FILTER_README document, where mail is filtered AFTER it is stored in the Postfix mail queue.

This document describes the following topics:

• Principles of operation
• Pros and cons of before-queue content filtering
• Configuring the Postfix SMTP pass-through proxy feature
• Configuration parameters
• How Postfix talks to the before-queue content filter