So you ask: What is tar pitting?

IMHO,  tar pitting is a very suitable way to stop misuse of your Windows 2003 / Exchange 2003 SMTP Virtual Server by delaying SMTP responses for 5.x.x errors. This efficiently stops DoS and harvesting attacks on your SMTP servers.

For example: Hacker A continously sends mail to your server, named yourserver.mail.net, the simple purpose being to gather valid e-mail-addresses from your domains. This Brute-Force-Attack is called a "Directory Harvest Attack". 

Have a look at this telnet-screenshot, where a simple SMTP-communication is depicted: 

Normally, this kind of communication would just take a view milliseconds of time on your SMTP server. With tar pitting enabled, SMTP Virtual Server will freeze the connection for a configurable  amount of time before returning "550 5.7.1 Unable to relay". More...