Graylisting

Part of speech: noun, verb

Definitions

1. A technique used in email filtering where messages from new senders are temporarily rejected to verify legitimacy ; a method of temporarily delaying email acceptance to deter spam senders ; a process in computer networking that imposes a waiting period on incoming communications to reduce unwanted or malicious traffic
2. An anti-spam strategy involving temporarily deferring emails from unknown sources to confirm authenticity ; a procedure of holding emails in a pending state before delivery to identify legitimate senders ; a network security measure that postpones acceptance of data to filter out suspicious or spammy messages
3. A system that temporarily rejects or delays emails from unknown senders to confirm they are not spam ; a preventive email control that imposes time restrictions to screen legitimate messages from unsolicited ones ; a computer-based approach that uses time-based deferral to help reduce the volume of spam by requiring repeated delivery attempts

Etymology: Graylisting is a technique for fighting spam, and its name comes from the greylist, a deliberately middle-ground list. A mail server that receives a message from an unfamiliar sender refuses it with a temporary error, and a legitimate server will simply try again later. Spam software often does not retry, so the message never arrives. The word is built from "grey" and "list" as a compound, with "graylist" as the base verb and noun, and "graylisting" is the form that describes the action or practice of doing this. The term plays on the older idea of a whitelist, a list of approved items, and a blacklist, a list of banned ones. The grey sits between them: senders are neither trusted nor banned, but held in a pending state until they prove they behave like a real mail system. The American spelling "graylist" is the usual form, while "greylist" appears in British usage.