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Postfix Documentation
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Postfix SMTP Access Policy Delegation

Purpose of Postfix SMTP access policy delegation

The Postfix SMTP server has a number of built-in mechanisms to block or accept mail at specific SMTP protocol stages. As of version 2.1, Postfix can delegate policy decisions to an external server that runs outside Postfix.

With this policy delegation mechanism, a simple greylist policy can be implemented with only a dozen lines of Perl, as is shown at the end of this document. Another example of policy delegation is the SPF policy server by Meng Wong at Examples of both policies can be found in the Postfix source code, in the directory examples/smtpd-policy.

Policy delegation is now the preferred method for adding policies to Postfix. It's much easier to develop a new feature in few lines of Perl, than trying to do the same in C code. The difference in performance will be unnoticeable except in the most demanding environments. On active systems a policy daemon process is used multiple times, for up to $ max_use incoming SMTP connections.

This document covers the following topics:

Postfix Documentation
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