Deliverability
Greylisting
An anti-spam technique that temporarily rejects emails from unknown senders, expecting legitimate servers to retry.
Definition
Greylisting is an anti-spam technique where a receiving mail server temporarily rejects emails from unknown senders with a "try again later" response (4xx error). Legitimate mail servers will retry delivery (typically within minutes), while many spam systems won't. This first-attempt rejection filters out significant spam volume. Greylisting can delay legitimate email delivery.
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