ME Enterprise has its own Bayesnean filter system that comes with default settings that you can modify. I have not messed with it nor read much on it, but their docs are pretty good. All other aspects of this software are pretty straight forward. Hope this helps.
Bayesian Filtering will takes some times to pick up the database as times goes by. If this is a new Mailenable box, I suggest taking a look on their docs http://www.mailenable.com/references.asp which shows you on setting up the "honey pot" and "ham" address to auto pick up mails for the BF database traininig.
what about a linux mail gateway sitting as your mx (incoming mail) and filter out most spams for you? good emails will then be forwarded back to your real email server. your real server should accept smtp traffic from the mail gateway only.