Hardening Postfix For ISPConfig 3

The goal of this tutorial is to harden the mail server postfix used
by ISPConfig for internet mail servers where authenticated users are
trusted. With this setup you will reject a great amount of spam before
it passes into your mail queue, saving a lot of system resources and
making your mail server strong against spammers and spam botnets.

HowtoForge – Linux Howtos and Tutorials – Linux