The Bayesian filter in SpamAssassin is one of the most effective techniques for filtering spam. Although Bayesian statistical analysis is a branch of mathematics, one doesn't necessarily need to understand the mathematics to use SpamAssassin's Bayesian filter.

Bayesian analysis involves teaching a system that a particular input gives a particular result. For Spam filtering, this teaching is repeated, many times over, with many spam and ham emails. Once this is finished, a Bayesian system can be presented with a new email and will give a probability of the result being spam. For best results, teaching should be a constant process.

To filter spam emails, the system is taught both ham and spam emails, until the filter has learned to differentiate between the two. Then, emails passed through the filter will be assigned a probability of being spam. When Bayesian filtering is used in conjunction with SpamAssassin's other spam detection rules, SpamAssassin approaches...