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Deliverability

Greylisting

Greylisting is a spam defence where a server temporarily rejects mail from an unfamiliar sender and accepts it on the retry. Real mail servers retry; most spam software does not.

For delivery it is harmless — it delays a first message by a few minutes and nothing else. For verification it is a nuisance, because a probe that gets a temporary rejection has learned nothing about whether the mailbox exists.

The correct handling is to treat a temporary rejection as unknown and check again later, rather than recording it as invalid. A verifier that collapses temporary failures into "invalid" quietly deletes a slice of perfectly good addresses from every list it touches.

Related terms

SMTP
SMTP is the protocol mail servers use to hand messages to each other. In verification, an SMTP probe opens a conversation with the receiving server and asks whether a specific mailbox exists, without sending anything.
Email verification
Email verification is checking whether an address can actually receive mail before you send to it. A full check tests the syntax, then the domain's mail records, then asks the receiving server about the specific mailbox.
Soft bounce
A soft bounce is a temporary rejection — a full mailbox, a server that is down, or a rate limit. The address may work later, so it is retried rather than removed.