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Definition: greylisting

A spam prevention method in which the receiving mail server rejects a message the first time it is received. The sending server of a legitimate user will eventually retransmit the message. However, the servers that spammers use generally will not because the spammer's goal is to disseminate millions of messages quickly and not have to keep track of which ones fail to reach their destination.

Based on the "Triplet"
When a message is received, the greylisting routine checks the IP and email addresses of the sender and the email address of the receiver (the "triplet"). If the triplet has been accepted before, the message is accepted again. If not, the receiving server sends a "busy" message to the sending server. See blacklist and spam filter.