Glossary · Deliverability
A hard bounce is a permanent delivery failure, such as a non-existent address or domain, signalled by a 5xx SMTP reply.
The receiving server rejects the message with a permanent error, typically 550 5.1.1 (user unknown), or the domain doesn't exist at all. Retrying won't help.
Remove hard-bounced addresses after the first failure. Repeatedly sending to addresses that don't exist is a classic sign of a bought or stale list and drags down sender reputation. Compare a soft bounce.
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