SpamAssasin - Leverages DNS block-lists of known spammers

Currently people are using content filtering with key words or phrases. This does'nt help when they send the message in an image. Nor does it stop the spam mastermind.
This tool accesses DNS block-lists on the Internet blocking known spam senders. It is open source too! SpamAssassin interacts with your mail server by analysing each email message using multiple methods of detection. Including integrity analysis and heuristic rules. These rules detect spam messages by testing all content and assigning score levels based on the messages spam or non-spam characteristics.

Yes this requires parsing and processing. But this tool can handle thousands of messages-its fast. It also has an "AutoWhiteList" that provides intelligence by adjusting the senders ratings based on good or bad spam messages over time. The "AutoBlackList" blocks the bad guys.

SpamAssassin also leverages third-party plug-ins and runs on Windows, Linux, and Mac OS X platforms.