Postfix is an open-source and free mail transfer agent (MTA). It is often used on Linux and Unix-like system to routes and delivers electronic mail. It also defaults on many Linux distros. It is also believed that pproximately 33% of the mail-servers on the Internet ran Postfix. Configuring anti-spam is a hard and important task for sysadmin.
From the tutorial page:
One of the most important and hardest tasks for every company that provides mail services is staying out of the mail blacklists. This article contains an analysis and solution for blocking of international SPAM botnets and a tutorial to install the anti-spam plugin to postfix firewall – postfwd in the postfix MTA.