Greylisting

One interesting phenom I ran across today is a spam filtering method called “Greylisting.”

Wikipedia describes Greylisting like this:

Greylisting…is a method of defending electronic mail users against e-mail spam. 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 most likely try again to send it later (see disadvantages), at which time the destination will accept it. If the mail is from a spammer, it will probably not be retried, and spam sources which re-transmit later are more likely to be listed in DNSBLs and distributed signature systems such as Vipul's Razor.