An anti-spam technique where a server temporarily rejects mail from unknown senders, expecting legitimate ones to retry.
Greylisting is a spam-fighting tactic. The first time a server sees a sender, it returns a temporary "try again later" error (a 4xx soft response) instead of accepting the mail. Real mail servers retry after a delay and get through; many spam tools never bother, so they are filtered out.
For verification, greylisting muddies the picture: an SMTP probe to a greylisting server gets a temporary deferral that says nothing about whether the mailbox exists. Treating that "try again" as either valid or invalid would be wrong.
The correct handling is to recognise the 4xx response, retry after the expected delay, and only then judge the address — which is exactly how a careful engine avoids false verdicts on greylisted domains.
A temporary delivery failure caused by a transient issue, such as a full mailbox or a server that is briefly unavailable.
Checking whether a specific mailbox exists by starting an SMTP conversation with the receiving server without sending a real email.
The standard protocol mail servers use to send and receive email across the internet.
See it in action
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