Hard bounce and soft bounce

A bounce is a delivery failure reported back to the sender. The distinction between hard and soft is the distinction between permanent and temporary, and it is drawn by the response code the receiving server returned.

A hard bounce is a 5xx: the mailbox does not exist, the domain accepts no mail, the recipient was rejected outright. Retrying changes nothing, and continuing to send to an address that hard-bounced is one of the clearest negative signals a mailbox provider can observe. Hard bounces are what verification exists to prevent — every one of them is an address that could have been identified before the send rather than after it.

A soft bounce is a 4xx: the mailbox is full, the server is busy, the message was greylisted, the recipient is temporarily unavailable. The address may be perfectly good. Mail systems retry soft bounces automatically for a period before giving up, and a soft failure that persists across a long retry window eventually becomes a hard one in practice.

Verification maps this onto its own verdicts, and the mapping is the point: a permanent rejection is undeliverable, a temporary one is unknown. Those are different claims. Undeliverable says mail will not arrive. Unknown says we could not establish whether it would. Collapsing the second into the first would mean discarding good addresses on the strength of a busy server — which is why a timeout is never reported as a failed address.