From Wikipedia, the free encyclopedia:
Greylisting is a simple method of defending electronic mail users against e-mail spam. In short, a mail transfer agent which uses greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again to send it later, at which time the destination will accept it. If the mail is from a spammer, it will probably not be retried, however, even spam sources which re-transmit later will be more likely to be listed in DNSBLs and distributed signature systems such as Vipul's Razor.
Greylisting requires little configuration and modest resources. It is designed as a complement to existing defenses against spam, and not as a replacement.