SPAM, greylisting, anti-SPAM measures, SMTP, Postfix, Postgrey

Greylisting is a method for protection against unsolicited electronic mail messages (SPAM) based on the combination of white lists and temporary deferring of incoming messages from new sources. The idea is quite simple and the method proved to be surprisingly efficient but there are some issues to be concerned with. One of the most important issues is possible efficiency decay through a long-term period. Therefore the investigation of the efficiency of the method throughout longer period was done Also various other factors of the greylisting method application were studied and are discussed.