The Jargon File (version 4.4.7, 29 Dec 2003):

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.