SMTP code 5.1.1
What the knowledge base's own tables say about this code, and which articles say it — a description of the reply, never a placement prediction.
The knowledge base documents this code in more than one row. What tells those rows apart is the rejection text in your own log, not the code: read each line below against what your rejection says.
- Field
- Status
- Required?
- Required
- Syntax and meaning
- Enhanced status code, DIGIT.1*3DIGIT.1*3DIGIT (for example 5.1.1)
From Delivery Status Notifications — DSN Format (RFC 3464) · Per-recipient fields
- Meaning
- The address does not exist, cannot be routed, or has gone
- Next step
- Hard bounce: suppress immediately. Repeated 5.1.1s are also how providers detect stale lists
From SMTP Enhanced Status Codes (RFC 3463) · Using the codes for bounce classification
- Test that tells causes apart
- Is it one address or many? One points to a typo or a churned address. Many point to a list-quality problem
- Next step
- Suppress the address (a hard bounce means permanent suppression). If many: audit the acquisition source and see list hygiene
- Rejection says…
- "user unknown", "mailbox does not exist", 5.1.1, 550 5.1.1
- Hypothesis
- Invalid recipient
From Delivery Troubleshooting Playbooks · Playbook A: message not delivered (bounced or rejected, 5xx)