PowerMTA.in resource
SMTP Error Codes: 4xx and 5xx Explained
SMTP responses are only part of the diagnosis. Always read the complete server response and review your sending logs before changing configuration.
421 SMTP Error
Service not available / temporary connection issue. Often temporary. Check the receiving server response, retry policy and connection rate.
450 SMTP Error
Mailbox unavailable or temporary failure. Check whether the recipient server is temporarily rejecting mail and retry according to its guidance.
451 SMTP Error
Local processing error / temporary failure. Review the full SMTP response and server logs. A temporary retry may succeed.
452 SMTP Error
Insufficient system storage / temporary limit. The receiving server may be temporarily unable to accept more mail. Retry later.
500 SMTP Error
Syntax error in command. Check the SMTP command sequence and client/server compatibility.
501 SMTP Error
Syntax error in parameters. Check addresses, command parameters and message formatting.
502 SMTP Error
Command not implemented. The server does not support the SMTP command being requested.
503 SMTP Error
Bad sequence of commands. The SMTP session is out of order. Check authentication and command sequencing.
550 SMTP Error
Mailbox unavailable / permanent rejection. Check recipient validity, policy response, authentication and sender reputation.
551 SMTP Error
User not local / forwarding response. The recipient may be hosted elsewhere. Follow the server response when appropriate.
552 SMTP Error
Exceeded storage allocation or message size limit. Check mailbox quota and message size limits.
553 SMTP Error
Mailbox name not allowed / address policy issue. Verify the recipient address and domain policy.
554 SMTP Error
Transaction failed / policy rejection. Read the complete response. Common causes include policy, reputation, authentication or content issues.
4xx vs 5xx
4xx responses are generally temporary and may be retried. 5xx responses are generally permanent for that transaction and require investigation before repeated attempts.
What to collect when troubleshooting
- Full SMTP response text
- Timestamp and recipient domain
- Sending IP and hostname
- SPF, DKIM and DMARC results
- Relevant MTA logs
- Whether the response is temporary or permanent