greylisting
also: grey listing, greylist, temporary rejection
Greylisting is an anti-spam technique that temporarily rejects incoming emails from unknown senders, allowing legitimate mail servers to retry while filtering out spam.
Greylisting works by maintaining a database of sender IP addresses, domains, and recipient addresses. When an email arrives from an unknown combination, the server returns a temporary rejection (SMTP code 4xx), telling the sender to try again later.
Most legitimate mail servers will retry delivery after a delay (typically 30 minutes to a few hours), and their credentials are added to a whitelist. Spam bots, however, often don't implement proper retry logic and give up immediately, so subsequent emails from that sender are accepted quickly.
For example, a mail server might greylist an email from [email protected] to [email protected] on first contact. When example.com's server retries after an hour, the tuple is whitelisted and future emails are delivered promptly. Meanwhile, most spam sources never retry and never make it to the inbox.