This article explains tar pitting your Windows 2003 Server.
Important This article contains information about how to modify the registry.
Click here for the full MS KB article (opens a new window).
What is SMTP tar pitting?
Tar pitting is the practice of deliberately inserting a delay into certain SMTP communications that are associated with spam or with other unwanted traffic. To be effective, these kinds of communications typically rely on generating a high volume of traffic. By slowing an SMTP conversation, you can dramatically reduce the rate at which automated spam can be sent or at which a dictionary attack can be conducted. Legitimate traffic may also be slowed by tar pitting.