Hard bounce vs soft bounce: what to retry and what to remove
A hard bounce is a permanent delivery failure; a soft bounce is temporary. That shorthand is useful, but the status code and diagnostic text—not the label in an email platform—should determine what you do next.
Updated July 15, 2026
The dependable distinction
| Signal | Meaning | Normal action |
|---|---|---|
| 5xx / 5.x.x | Permanent failure | Fix or suppress before resending |
| 4xx / 4.x.x | Temporary failure | Allow controlled queue retries |
| 2xx / 2.x.x | Success | Investigate a later event or inbox placement if mail is missing |
SMTP reply codes use the first digit as the broad outcome. Enhanced status codes add a subject and detail, such as 5.1.1 for an invalid recipient or 4.2.2 for a temporarily unavailable full mailbox. A provider can still add proprietary text, so read the entire diagnostic.
Common hard bounces
- Invalid, deleted, or misspelled recipient (
5.1.1). - Message too large for the destination (
5.3.4) until you change the message. - Permanent authentication or policy rejection (
5.7.x) until the sender fixes the stated cause. - A recipient domain or route that permanently does not exist.
Common soft bounces
- Mailbox temporarily full (
4.2.2). - Remote server unavailable or DNS lookup temporarily failing.
- Rate limiting or greylisting.
- Temporary reputation or policy deferral.
Your sending server should retry with backoff. Set a finite retry window, then stop and investigate recurring failures instead of creating a permanent retry loop.
A safe decision process
- Paste the complete notice into the email bounce analyzer.
- Confirm whether the first digit is 4, 5, or 2.
- Use the enhanced code and nearby prose to identify recipient, routing, authentication, policy, reputation, or content as the cause.
- Suppress invalid recipients immediately; retry temporary failures with backoff; remediate authentication or policy failures before another send.
- Track repeated bounces by category without storing more message content or personal data than necessary.