Gmail 451 4.5.0
SMTP protocol violation
Gmail 451 4.5.0 means: sMTP protocol violation. It is a temporary failure, so your server retries it. Your client broke the SMTP protocol in a way Gmail treats as temporary. Check the sending software against RFC 5321.
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
SMTP protocol violation. For more information, go to About SMTP error messages and review RFC 5321 specifications.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 451: Requested action aborted: local error in processing (RFC) |
| Enhanced code | 4.5.0: Other or undefined protocol status |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
What the sender should do
Your client broke the SMTP protocol in a way Gmail treats as temporary. Check the sending software against RFC 5321.
Would verifying the list have prevented Gmail 451 4.5.0?
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 451 4.5.0 mean?
- Gmail 451 4.5.0 means: sMTP protocol violation. It is a temporary failure, so your server retries it. Your client broke the SMTP protocol in a way Gmail treats as temporary. Check the sending software against RFC 5321.
- Should I retry after Gmail 451 4.5.0?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Gmail 451 4.5.0?
- 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.
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