Try G-Lock Spamcombat [broken link removed]
Takes a bit of training but is very good and free. I don't use many free services but this is one I recommend.
Can blacklist by country, ip, subject, domain, sender etc.
Been using it for about 3 years. Spam goes to a deleted items folder prior to hitting your server and allows you to save to disk and move to recoverable items for genuine mails that can come in from time to time from non-white listed addresses.
I've mine set to check every few mins and move suspect mails to the trash (deleted items folder). Any serious dodgy mails are set to permanantly delete off server prior to downloading them onto Win Mail.