ONLamp.com: Spam Filtering with Sendmail Milters and Greylisting
With spam, viruses, worms, and trojan horses consuming an ever-increasing amount of mail traffic, mail filtering is more important than ever. So is mail server flexibility. Sendmail's milters allow administrators and programmers to control almost every step of the mail process. Emmanuel Dreyfus explains how to write a milter by demonstrating his anti-spam greylisting system.