Definition - What does Tarpitting mean?
Tarpitting is a network security and optimization process through which network administrators (NA) intentionally slow down the propagation of mass emails by restricting and demotivating spammers from sending bulk messages.
This process is derived from a server, Teergrube ("tar pit" in German), which prevents spammers from using/connecting to a server by deliberately granting access to all new requesting users or machines.
Techopedia explains Tarpitting
Tarpitting allows network and server administrators to maintain a high degree of network operation and maximum bandwidth availability. It works when a server identifies a client's abnormal email operations. Typically, such behavior is exhibited by spammers that have thousands of emails to send in a very limited period. When a NA/server identifies this type of user/client, it slows down or "tarpits" the process, eventually preventing spammers from succeeding.
Spammers also use tarpitting to reverse the process in such a way to prevent detection by a monitoring user/application/server. For example, to avoid being tarpitted, a spammer may send bulk emails in short batches over a relatively longer period than usual.