"Tarpitting" also known as 'Teergrubing' is a way of reducing spam or spam-related attacks (such as directory harvesting). It involves delaying SMTP communications with remote servers suspected of sending unsolicited e-mail. This slows down the the automated email sender threads of spammers as they are forced to spend more time to send out the email and in some cases give up all together as their session timesout.

There are a lot of variants of this, some servers might tarpit based on sender reputation, while others look out for number of concurrent connections made, number of recepients addressed in the RCPT TO field of the same connection etc.

Metrics which show the average smtp session time and number of time smtp session timeouts per email domain might be a good way to find out if you are being tarpitted by a particular domain.