Chris Hardie's Qmail Anti-Spam HOWTO is a good resource. You will find this to be quite thorough. Georg Lehner has written "How to use Spamassassin together with Qmail." In it Georg describes how to setup Qmail with Spamassassin to meet your needs. You should also checkout Vipul's Razor: Vipul's Razor is what you now commonly know as SpamNet. For nearly two years, Razor has been successfully fighting spam with the help of the Unix community and is the technology that has enabled us to build its windows counterpart, SpamNet, currently in use by more than 200,000 users. Razor, or SpamNet, is a distributed, collaborative, spam detection and filtering network. It establishes a distributed and constantly updating catalogue of spam in propagation. This catalogue is used by clients to filter out known spam. Razor2/SpamNet use a variety of signature algorithms to detect polymorphic spam. A trust metric known as TeS mediates all collaborative interaction and decides what spam makes it to the catalogue.