The Book of PF, 2nd Edition

Giving Spammers a Hard Time with spamd

Email is a fairly essential service that needs special attention due to the large volume of unwanted messages, or spam. The volume of unsolicited commercial messages was already a rather painful problem when malware makers discovered that email-borne worms would work and started using email to spread their payload. During the early 2000s, the combined volume of spam and email-borne malware had increased to the point where running an SMTP mail service without some sort of spam countermeasures had become almost unthinkable.

Spam-fighting measures are almost as old as the spam problem itself. The early efforts focused on analysis of the messages’ contents (known as content filtering) and to some extent on interpretation ...