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.The tar pit feature is available in Microsoft Windows Server 2003 and in several third-party SMTP servers. The tar pit feature in Windows Server 2003 works by slowing all responses that contain SMTP protocol 5.x.x error codes.
Tar pitting is a feature of Windows 2003 so Exchange 2000 can benefit too.
http://support.microsoft.com/default.aspx?kbid=842851