Greylisting is a spam filtering method that consists in blocking the initial attempt to receive a new message. GateWall Antispam generates a list of triplets including the IP address originating a connection, the address received in MAIL FROM command and the address specified in RCPT TO command. A message is qualified as new mail if its triplet has never been received before. The message is blocked, and a "temporary error" notice is sent. When a sender's server receives a "temporary error" notice, it is supposed to retry sending the message later. Greylisting settings specify triplet storage time and exceptions lists.