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SMTP tar pit feature for Microsoft Windows Server 2003

Tarpitting 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. An administrator can configure the delay that is introduced by the tar pit feature.

Tar pitting affects only anonymous SMTP connections. Authenticated sessions are exempt. Therefore, if you regularly exchange lots of SMTP mail with another organization,
and you find that tar pitting is affecting that traffic, you can bypass tar pitting for that organization by authenticating SMTP communications.

More details you can find here: