You could take a look at POPFile which is one of the Bayesian stats based filters. It's name makes it sound like it requires POP3 to work, but someone has come up with an Outlook add-in that then plugs into POPFile, meaning that you can use it with Exchange.

The main POPFile site is here:

http://popfile.sourceforge.net/

The details on the Outlook add-in are here:

http://sourceforge.net/forum/forum.php?thread_id=801506&forum_id=226152

N.B. I have not used POPFile myself for more than a day or so, because I do my spam filtering on my server using Bogofilter http://bogofilter.sourceforge.net/