Tarpitting is the practice of slowing the transmission of e-mail messages sent in bulk as a means of thwarting spammers. The intent is to maintain a high quality of service for legitimate users while making the sending process impractical for spammers, who -- because of low response rates -- must be able to send vast volumes of messages quickly and inexpensively. The delay is insignificant for typical recipient lists, and administrators can grant exemptions to people with valid reasons to send messages to a large number of recipients.
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There are a number of approaches to tarpitting. One method is to insert small delays (sometimes called sleeps) after a certain number of recipients in a Simple Mail Transfer Protocol (SMTP) session. For example, the first 20 messages in a session might be sent without delay and then a few seconds' delay added for each recipient in the list beyond that number. If a spam mailing list contains 10,000 recipients and a delay of even two seconds is applied for each recipient after the twentieth, the total delay incurred is over five and a half hours.