From The Office Maven:
Junk-Out keeps junk mail out of your Microsoft Outlook Inbox, using a combination of statistical methods (Bayesian filtering) and information you provide, to show only e-mail messages you want to see. The more you use Junk-Out, the better it gets at distinguishing useful e-mail, and getting junk out of your way. The heart of Junk-Out is its continually learning content-based / Bayesian filter mechanism. This feature can use a Fast Track word list, a seed vocabulary of over 30,000 words, to start filtering junk out of your incoming e-mail as soon as you install it. Then it gets better with every e-mail you receive, adding to its knowledge from experience and the training you give it with just a click of the mouse. Version 1.14.0048 works with Outlook 2003 (as well as 2002 and 2000) and has many new features.