Greylisting.

An anti-spam technique that initially returns a temporary SMTP rejection for unknown sender triplets and only accepts the message on a later, properly retried delivery attempt. It belongs to the Network Security category of cybersecurity.

An anti-spam technique that initially returns a temporary SMTP rejection for unknown sender triplets and only accepts the message on a later, properly retried delivery attempt.

Greylisting, popularized by Evan Harris in 2003 and described in RFC 6647 in the broader context of SMTP anti-spam, exploits the fact that compliant Mail Transfer Agents queue and retry messages after a 4xx temporary error, while many spam senders do not. The receiver records a triplet of (sending IP, MAIL FROM, RCPT TO); on first sight it responds with 451 4.7.1, then accepts the message once a retry arrives after a configurable delay. Once a triplet is trusted, future messages bypass the delay. Modern deployments narrow scope with reputation feedback, exempt known senders, integrate with DNSBLs, and consider trade-offs because legitimate cloud platforms may use short-lived IPs that complicate retry alignment.

Defences for Greylisting typically combine technical controls and operational practices, as detailed in the full definition above.

Common alternative names include: Grey Listing, Tempfail Greylisting.