Gmail 554 5.6.0
Message malformed
Gmail 554 5.6.0 means: message malformed. It is a permanent failure, so do not retry it unchanged. The message is malformed under RFC 5322. Check headers and MIME structure from the sending tool.
Gmail’s exact wording
Quoted from Google Workspace Admin Help: Gmail SMTP errors and codes, checked 2026-11-05. Placeholders are Gmail’s own.
Reply text
Email message is malformed. Not accepted. For more information, go to Email sender guidelines and review RFC 5322 specifications.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 554: Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here") (RFC) |
| Enhanced code | 5.6.0: Other or undefined media error |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
Malformed messages usually come from code that builds MIME by hand, for example wrong boundaries, missing headers or invalid encodings. Send the same message to a test inbox and look at the raw source, or use a standard mail library to build the message. Rebuilding the message with a maintained library usually clears the error on the next attempt.
What the sender should do
The message is malformed under RFC 5322. Check headers and MIME structure from the sending tool.
Would verifying the list have prevented Gmail 554 5.6.0?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Gmail 554 5.6.0 mean?
- Gmail 554 5.6.0 means: message malformed. It is a permanent failure, so do not retry it unchanged. The message is malformed under RFC 5322. Check headers and MIME structure from the sending tool.
- Should I retry after Gmail 554 5.6.0?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. The message is malformed under RFC 5322. Check headers and MIME structure from the sending tool.
- Would verifying my list have prevented Gmail 554 5.6.0?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
The same code elsewhere
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05