Well, this is not free (so I'm not much of a help at that) but after dealing with spam issues I forked out the necessary bucks and purchased SpamSieve...worth every penny I spend on it.
Spam issues can get very serious, specially if you value your time...and you can spend lots of it sorting your mail and making sure some important mails don't get trashed as junk. My extensive search for freeware anti-spam software got me nowhere near any software that was good enough as the commercial ones...and SpamSieve clearly came on top of them all.
HTH