From Jargon File (4.3.1, 29 Jun 2001) :
teergrube /teer'groob/ n. [German for `tar pit'] A trap set to punish
spammers who use an address harvester; a mail server deliberately set
up to be really, really slow. To activate it, scatter addresses that
look like users on the teergrube's host in places where the address
harvester will be trolling (one popular way is to embed the fake address
in a Usenet sig block next to a human-readable warning not to send mail
to it). The address harvester will dutifully collect the address. When
the spammer tries to mailbomb it, his mailer will get stuck.