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Anti-spam technique: Tarpitting
Date of first use: late 1990s
Effectiveness: Low
Popularity: Low
Difficulty of implementation: Medium
Where implemented: MTA
Harm: Low

Tarpitting involves keeping SMTP clients online as long as possible, on the theory that while they are stuck in the tarpit they won't be delivering spam to someone else. Tarpits were modestly effective when spamware was single threaded, but now that spam is typically delivered through farms of botnets, and both legitimate and illegitimate MTAs are typically multithreaded, there's little reason to think that they have any effect any more.

Some of the most sophisticated tarpits were reported to keep up to 4,000 simultaneous sessions active, and to keep some clients online for as much as several days by slowly feeding them endless numbers of server message continuation lines.

Some early tarpit software was developed in Germany, so the technique is also known by the German name teergrube.