This HOWTO, from Howtoforge.com provides a great look into how to leverage the best defense against spam on your mail server. Great overview with setp by step instructions:
This document describes how to integrate ASSP (Anti-Spam SMTP Proxy) with embedded ClamAV into a mail server based on Postfix featuring virtual users and domains, i.e. users and domains that are in a MySQL database.
HOWTOForge is a great place to pick up some great guides on setting up your server.
This guide describes how to set up a full email solution in Debian Linux (all code is from Debian Etch). I was asked to design a secure, scalable, portable solution for a small company.
He goes through every stage, from soup to nuts, of all the issues and steps involved in getting your mail up to the level you want it at.