Greylisting is a spam-defence technique in which a receiving server temporarily rejects email from an unfamiliar sender and asks it to try again shortly afterwards.
In depth
On first contact the receiver replies with a temporary 4xx error. A properly configured, legitimate mail server automatically retries after a delay and is then accepted; many bulk spam tools never retry, so they are filtered out. The trade-off is a delivery delay of minutes to an hour on the very first message from a new sender to a given recipient. After the sender is known, subsequent messages pass without delay.
Example
The first email from a new sending IP might arrive 15 minutes late while the receiving server waits for the retry; the second email arrives instantly.
Related terms
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