Agnitum Spam Terrier is a powerful tool that can be integrated into Outlook and Outlook Express; it analyzes each email you receive to determine whether it's spam.
You can choose to trust Agnitum Spam Terrier's automatic detection system or set up criteria for letting an email into your inbox or adding it to a blacklist of senders.
In addition, the latest version of the program includes support for The Bat! and Windows Mail. Keep in mind that future versions of the application will probably be compatible with even more email clients.