SMTP bounce tool
SMTP Error Code Lookup
Paste a bounce response to see whether you should retry, suppress the recipient, fix sender authentication, or investigate further.
SMTP diagnosis will appear here after you analyze a bounce response.
About this SMTP error lookup
This tool checks provider, SMTP reply code, enhanced status code, remote host, and diagnostic text. It is designed to prevent the common mistake of suppressing a real recipient when the provider actually rejected sender authentication, policy, routing, or rate limits.
How to use
- Paste the full SMTP reply or bounce diagnostic text.
- Add the provider or remote host if the response text is short.
- Review the recommended action before retrying or suppressing.
What the results mean
- Retry unchanged means the same message can be queued again without changing sender or recipient data.
- Suppress recipient should only be used when the provider-specific evidence says the mailbox is invalid.
- Fix sender setup means SPF, DKIM, DMARC, DNS, routing, content, or policy should be corrected before retrying.
Browse SMTP error guides
Use the indexed guides when you know the SMTP code. Use this lookup when you have the full bounce response.
Open SMTP error guides