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.