A couple of people recently have mentioned that they tried out some free utilities that I recommended, so I'll recommend another one of my favourites.
K9 is a email spam filter, written by Robin Keir. Being a bayesian filter, it needs a little bit of training initially, but once it has been told which emails are good, and which are bad, then it does a really good job of sorting the wheat from the chaff. On my system, it's currently running at 99.19%. The remaining 0.81% seem to be false negatives, rather than false positives, so I don't miss any emails that I shouldn't.
There's whitelist and blacklist support, which I find really handy, as I have all email addresses for all my domains coming in through it, so I can blacklist random addresses, such as [email protected], sales@, [email protected] etc., and always let mails through from my friends, no matter what nonsense they're on about (actually, one or two might get blacklisted soon, if they don't stop sending me jokes and hoax virus warnings ;-] ).
Anyway, I've been using this for over a year, and I'm delighted with it, and recommend you give it a go.