A technique used by some Mailbox Providers and email receivers to thwart spammers. A receiving mail server using greylisting will temporarily reject any email from a sender it does not recognize. The receiver presumes that if the sender is legitimate, the originating server will most likely try again to send it later at which time the receiver will accept it. Greylisting presumes that if the sender is a spammer, they will not retry later to transmit their message.
Greylisting has disadvantages and is somewhat controversial.

A technique used by some Mailbox Providers and email receivers to thwart spammers. A receiving mail server using greylisting will temporarily reject any email from a sender it does not recognize. The receiver presumes that if the sender is legitimate, the originating server will most likely try again to send it later at which time the receiver will accept it. Greylisting presumes that if the sender is a spammer, they will not retry later to transmit their message.
Greylisting has disadvantages and is somewhat controversial.