Qmail Support Forum

admin · Site Admin Joined: 21 Jun 2005 Posts: 209

What's tarpitting? It's the practice of inserting a small sleep in an SMTP session for each RCPT TO after some set number of RCPT TOs. The idea is to thwart spammers who would hand your SMTP server a single message with a long list of RCPT TOs. If a spammer were to attempt to use your server to relay a message with, say, 10,000 recipients, and you inserted a five-second delay for each recipient after the fiftieth, the spammer would be "tarpitted," and would most likely assume that his connection had stalled and give up.

http://www.palomine.net/qmail/tarpit.html