Gmail 501 5.5.2
Cannot decode response
Gmail 501 5.5.2 means: cannot decode response. It is a permanent failure, so do not retry it unchanged. Gmail could not decode the client's response, usually during authentication. Check the AUTH encoding your client uses.
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
Syntax error, cannot decode response. For more information, go to About SMTP error messages.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 501: Syntax error in parameters or arguments (RFC) |
| Enhanced code | 5.5.2: Syntax error |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
What the sender should do
Gmail could not decode the client's response, usually during authentication. Check the AUTH encoding your client uses.
Would verifying the list have prevented Gmail 501 5.5.2?
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 501 5.5.2 mean?
- Gmail 501 5.5.2 means: cannot decode response. It is a permanent failure, so do not retry it unchanged. Gmail could not decode the client's response, usually during authentication. Check the AUTH encoding your client uses.
- Should I retry after Gmail 501 5.5.2?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. Gmail could not decode the client's response, usually during authentication. Check the AUTH encoding your client uses.
- Would verifying my list have prevented Gmail 501 5.5.2?
- 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