Hard bounce
Permanent delivery failure (typically 5xx) because the address or domain cannot accept mail.
Problem summary
Permanent delivery failure (typically 5xx) because the address or domain cannot accept mail.
Symptoms
- Immediate 5xx responses such as user unknown
- DSNs classify failures as permanent
- Bounce rate jumps after a list import
Possible causes
- Invalid, mistyped, or role addresses
- Domain does not accept mail (NXDOMAIN / no MX)
- Mailbox deleted or disabled
How to diagnose
- Parse DSN status codes and diagnostic text
- Validate a sample of failing domains for MX
- Check whether failures cluster on a recent acquisition source
How to fix
- Suppress permanent failures immediately
- Stop mailing unclean lists; re-permission where required
- Fix domain-level DNS if your own domains are bouncing inbound
How to verify
- Hard-bounce rate returns below policy thresholds
- No repeated sends to known permanent failures
Prevention
- Verify addresses at capture
- Sunset chronically inactive addresses carefully
When to escalate
- Production mail is failing for a material share of recipients after remediation attempts