Hard bounce
Permanent delivery failure (typically 5xx) because the address or domain cannot accept mail.
- Reading time:
- 5 min
- Difficulty:
- Beginner
- Estimated duration:
- 5 min
- Prerequisites:
- None
- Last updated:
- 2026-07-15
- Author:
- empire
Problem summary
Permanent delivery failure (typically 5xx) because the address or domain cannot accept mail.
Severity: High
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
Related Learn Articles
Related Problems
- Deferred messagesReceivers return temporary SMTP failures (4xx), forcing retries instead of immediate acceptance.
- Delayed deliveryMail eventually arrives but takes much longer than normal due to greylisting, queues, or throttling.
- Emails not deliveredMessages never reach the recipient mailbox — rejected, discarded, or stuck without a successful delivery.
- Soft bounceTemporary delivery failure that may succeed later — full mailboxes, greylisting, or transient outages.