Locking Filter: it can block mail with text and string traps, scan the subject line, don't allow mail from some IP addresses, hosts or domains. Block mail to former employees and many more options
It can also block messages with attachments
Bayes Filter: It reads all messages and rates
them according to the good and bad words in its database. The Bayes filter builds its word database from the Spam the block filters. It can be fed by sending messages to it directly.
Heuristic Filter: a.k.a. Spam
Assassin can be implemented right out of the
box. This filter calculates the possibility that the message is spam by considering a number of factors like words, subject, headers and more.