Graylisting is a process for spam reduction or filtering that involves the strategic temporary rejection of a sender based on that sender's IP address and additional routing information. A server that uses a graylisting strategy will take in the sender's IP address and other routing information and attempt to match it against a database. If there is no match, the server will issue a temporary rejection error code. Graylisting is based on the notion that a legitimate server will receive the rejection and send the message again, while a spamming server is likely to write off the recipient's email address as invalid.
Graylisting uses a protocol called Simple Mail Transfer Protocol (STMP), which has become a popular standard for email handling.
Techopedia explains Graylisting
Concerns about graylisting are twofold: first, critics argue that spammers can easily get around this strategy by programming spamming servers to retry a sent message when it's rejected. Second, and perhaps more importantly, graylisting can effectively destroy the utility of email as an instant means of communication if it ends up rejecting all or most messages for a long period of time.