As of early in the Mozilla 1.3 alpha development cycle, Mozilla Mail now has basic junk-mail classification capabilities. Some parts of the functionality are not yet turned on, and there is a great deal of work yet to do. Keep this in mind if you decide to start playing with this functionality.
The infrastructure is being written with the idea of one day being able to have and use multiple mail filtering plugins. For the moment, however, only one is currently supported, and it does Bayesian classification as described in Paul Graham's A Plan for Spam. Using Bayesian filtering requires that you first train the mailer by showing it a bunch of mail that is junk, and a bunch of mail that is not. Then, you let it autoclassify new mail for you, and you continue to correct it as it makes mistakes.Links:
Initial Mozilla implementation work by beard, bienvenu, dmose, ducarroz, jglick, naving, nhotta, peterv, and sspitzer.