If I had a list (and maybe I should compile one) of absolute-must-have Mac apps, Command-C's Spam Sieve would certainly be in the top 5... maybe even the top 2.

SpamSieve uses Bayesian spam filtering to monitor incoming email from popular email clients and rid your inbox of spam. It's smart--it learns what your spam looks like, so it can block nearly all of it. It also looks at your address book and learns what your good messages look like, so it won't confuse them with spam. SpamSieve doesn't delete any messages--it only marks them in your email client--so you'll never lose any mail.

I've been using Spam Sieve for almost 4 years now and the tactics spammers use to foil the common filters have changed over that period of time. Methods used by other spam filters (including the Junk Mail filer in Apple's own Mail.app) have been rendered effectively useless agains the modern spammer, but Spam Sieve remains in control. I need no other filter than Spam Sieve.

Recently, C-Command released Spam Sieve 2.6.3 with various accuracy improvements, particularly for messages with attachments. It also has improved compatibility with pre-release versions of Mac OS X 10.5 Leopard. C-Command fixed a bug where the Software Update checker didn't always re-check for new updates (so you might need to manually check for this update).

Spam Sieve sells for USD$30.00--a bargain when you consider the email processing time you'll save. Just get it.