SpamSieve

SpamSieve gives you back your inbox by bringing powerful Bayesian spam filtering to popular e-mail clients.

It learns what your spam looks like, so it can block nearly all of it. It looks at your address book and learns what your good messages look like, so it won't confuse them with spam. Other spam filters get worse over time as spammers adapt to their rules; SpamSieve actually gets better over time as you train it with more messages. SpamSieve doesn't delete any messages--it only marks them in your e-mail client--so you'll never lose any mail. SpamSieve works with any number of mail accounts, of whatever types are supported by your e-mail software (e.g. POP, IMAP, Exchange, Hotmail, AOL).