550 5.1.0
550 5.1.0 User Unknown
Gmail rejected the message because the user part of the address is unknown.
Workspace administrators can remove or rename users without warning. Gmail then returns 5.1.0. Continued retries harm your sender score.
Severity
ERROR
Provider
Gmail
Reference
Mitigation Portal
Last Reviewed
2025-06-01
How to Fix 550 5.1.0
Follow these steps to resolve this Gmail SMTP error and restore normal delivery.
- ✓Flag the subscriber record and request an updated address.
- ✓Remove catch-all rules that mask user churn.
- ✓Implement bounce categorisation so user-unknown events are not retried.
- ✓Avoid sending follow-up campaigns to secondary aliases if the primary user fails.
Helpful Resources
More Gmail SMTP Codes
Keep these related troubleshooting guides handy while addressing 550 5.1.0.
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