Tarpitting

A technique used by spammers to slow down the transmission rate of unsolicited emails (spam) that are sent in bulk. For example, If the spammer has 10,000 emails to send, they may send the messages in groups of 25 at a time adding a delay in an SMTP communication between mail servers. When delaying the sending of mass numbers of email messages, the spammer is more likely to avoid having his or her spam behavior detected by an ISP.