Tarpitting

What it means

A tarpit holds back responses so that a client trying many addresses or passwords is slowed down on every attempt. Microsoft Exchange has it built in: a receive connector's TarpitInterval delays SMTP responses to a remote server that might be abusing the connection, 5 seconds by default, and AuthTarpitInterval does the same after failed authentication attempts. Microsoft warns that values above a few seconds can cause timeouts and mail flow problems.

To a sender a tarpit shows up as slow sessions and timeouts rather than as rejections, so a sudden drop in throughput to one destination with no change in replies can be the sign.