Ubuntu 9.10 Course: Secure The Ubuntu Mail Server

Hardening Postfix For ISPConfig 3
HowtoForge: "The goal of this tutorial is to harden the mail server postfix used by ISPConfig for internet mail servers where authenticated users are trusted."

Haraka Mail Server: Spamassassin Plugin
BeginLinux: "Spamassassin or similar tool is an absolute necessity for any mail server."

How To Install, Secure, And Automate AWStats...
HowtoForge: "AWStats is a free and very powerful tool that generates advanced web, streaming, ftp or mail server statistics, graphically. It can analyze log files from all major server tools...