Glossary
Hard bounce
Industry term, not a standards term, for a delivery failure that will not succeed on retry. It corresponds to RFC 3463's permanent failure class, 5.X.X.
In plain terms
A permanent failure. The address should come off your list.
Why it matters for deliverability
Hard bounce rate is the metric mailbox providers read as a proxy for how carefully you collect and maintain addresses. It is also the metric most distorted by mis-classifying full mailboxes as permanent failures.
What to do about it
- Suppress on the first occurrence — but only where the enhanced status code shows the mailbox itself is invalid.
- Classify strictly from the 5.X.X enhanced status code, not from bounce-message wording, and keep policy and authentication rejections (5.7.x) out of your hard-bounce metric.
- Verify lists before a send, not after — a hard bounce has already cost you the reputation hit.
Source
RFC 3463 — Enhanced Mail System Status Codes (basis for the 5.X.X permanent class)
“5.XXX.XXX Permanent Failure — A permanent failure is one which is not likely to be resolved by resending the message in the current form.”
Checked 2026-08-29
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Definitions reviewed 2026-08-29. Spotted something wrong? Tell us.
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