Learn how Outlook 2003's junk mail filter works in "Microsoft Outlook 2003 Spam Filter: Under the hood" by MAPILab. The author describes in detail how Microsoft Outlook 2003's junk mail filter works. This article is informative for anyone interested in antispam technologies, whether they use Outlook 2003 or other anti-spam programs.
http://www.mapilab.com/articles/outlook_spam_filter.html (article removed for legal reasons.)
MAPILab also has a number of interesting and useful Outlook Addins available.