Gmail 502 5.5.1
Unrecognized or unimplemented command
Gmail 502 5.5.1 means: unrecognized or unimplemented command. It is a permanent failure, so do not retry it unchanged. Your client sent a command Gmail does not support. Use standard ESMTP commands only.
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
Unimplemented command. For more information, go to About SMTP error messages.
Reply text
Unrecognized command. For more information, go to About SMTP error messages.
Reply text
Too many unrecognized commands, goodbye. For more information, go to About SMTP error messages.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 502: Command not implemented (RFC) |
| Enhanced code | 5.5.1: Invalid command |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
What the sender should do
Your client sent a command Gmail does not support. Use standard ESMTP commands only.
Would verifying the list have prevented Gmail 502 5.5.1?
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 502 5.5.1 mean?
- Gmail 502 5.5.1 means: unrecognized or unimplemented command. It is a permanent failure, so do not retry it unchanged. Your client sent a command Gmail does not support. Use standard ESMTP commands only.
- Should I retry after Gmail 502 5.5.1?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. Your client sent a command Gmail does not support. Use standard ESMTP commands only.
- Would verifying my list have prevented Gmail 502 5.5.1?
- 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