Graylisting

Policy Patrol Spam Filter for Exchange Server and Lotus Domino includes a graylisting system (also known as greylisting) that initially rejects emails from new senders for 2-3 minutes. Since regular mail servers will try to resend the message, their messages will still be delivered. However since spam engines are not proper mail servers and designed only to send out as much mail as possible, they will not try to resend the mail and therefore never get through. Graylisting can also stop virus outbreaks that are being spread from zombie machines since these use primitive mail sending systems that fail to resend the message upon initial rejection.