bogofilter

Fast Anti-Spam Filtering by Bayesian Statistical Analysis

Bogofilter is a Bayesian spam filter. In normal operation, it takes an e-mail message or text from standard input, does a statistical check against lists of "good" and "bad" words, and returns a status code indicating whether the message is spam. Bogofilter is designed with fast algorithms (including the Berkeley DB system), coded directly in C, and tuned for speed, so it can be used for production by sites that process a lot of mail.