[ excerpt with modifications from developer's site ]

The SpamBayes project is working on developing a Bayesian anti-spam
filter.

SpamBayes will attempt to classify incoming email messages as 'spam',
'ham' (good, non-spam email) or 'unsure'. This means you can have
spam or unsure messages automatically filed away in a different
mail folder, where it won't interrupt your email reading.

First, SpamBayes must be trained by each user to identify spam and
ham. Essentially, you show SpamBayes a pile of email that you like
(ham) and a pile you don't like (spam). SpamBayes will then analyze
the piles for clues as to what makes the spam and ham different.
For example; different words, differences in the mailer headers and
content style. The system then uses these clues to examine new
messages.

WWW: http://spambayes.sourceforge.net/