Hard bounce
A permanent delivery failure for a reason that will not change on retry, typically because the address or its domain does not exist.
What it means
Hard bounces come back with a 5xx reply and usually an enhanced status code in the 5.1.x range: 5.1.1 for a mailbox that does not exist, 5.1.2 for a destination domain that cannot receive mail, 5.1.10 for a domain with a null MX. The term is industry usage, not an RFC definition, and platforms draw its edges differently; Postmark, for example, defines a hard bounce by examples such as an unknown user or a mailbox not found.
The right response is to stop mailing the address at once. Hard bounces count against senders: Amazon SES counts them toward the bounce rate that can put an account under review, and Bird, for one, adds a hard-bounced address to its suppression list automatically.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 3463, section 3.2: Address Statusrfc-editor.org
- Postmark API: Bounce typespostmarkapp.com
- Amazon SES Developer Guide: Enforcement FAQs (bounce rate)docs.aws.amazon.com
- Bird Docs: Why did my email bounce?bird.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes