Bounce

What Bounce means

A bounce begins as an SMTP reply code. A 5xx reply is permanent — the address does not exist, the domain accepts no mail, the sender is refused — and a conforming sender must not retry it. A 4xx reply is transient: a full mailbox, greylisting, a rate limit, a temporary DNS failure. The sender keeps the message queued and retries, typically for four or five days before giving up.

What arrives in a mailbox afterwards is a delivery status notification, formatted per RFC 3464 and carrying an enhanced status code from RFC 3463 — 5.1.1 for an unknown recipient, 5.7.1 for a policy rejection, 4.2.2 for a full mailbox. The notification is returned to the envelope sender rather than to the header From, and is itself sent with an empty envelope sender, written <>, so that a bounce can never bounce in turn.

A hard bounce has to be suppressed on the first occurrence. An address that answered 5.1.1 will answer 5.1.1 forever, repeated attempts are visible to the receiver, and a dead address left on a list is exactly what later becomes a recycled spam trap. Transient replies deserve the opposite treatment: read the text, because a 4.7.x code that names a policy is often a blocklist rejection or an authentication failure wearing a temporary code.