I have heard that the sonicwall email security appliance is pretty good. It gets expensive per user, but they have desktop controls in outlook.
The other one is the service offered by mcaffee enterprise... I don't remember the name, but its essentially a service they host and your mail server only receives mail from them. Its underlying system is spamassassin, but theirs is on steroids. I think the one of the developers of spamassassin actually works for them. Both systems use Bayesian filtering, but they have different types of input that allow millions of indicators to be used and updated minute by minute, thus allowing maximum spam detection. Basically input from many places is always better than input from one place.. The more data you have to look at the more obvious the trends thus making it easier to spot spam and rule out false positives on a minute by minute basis. -Brent
