X.5.2 · Mail delivery protocol
SMTP enhanced status code 5.5.2
Syntax error
X.5.2 means the server could not interpret a command, because the syntax was wrong or the command is unknown.
| Registry code | X.5.2 |
|---|---|
| Forms in use | 5.5.2 |
| Registry name | Syntax error |
| Subject | 5: Mail delivery protocol |
| Permanent or temporary | 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 500, 501, 502, 550, 555 |
| Defined in | [RFC 3463] |
What the registry says
A mail transaction protocol command was issued which could not be interpreted, either because the syntax was wrong or the command is unrecognized. This is useful only as a permanent error.
IANA SMTP Enhanced Status Codes registry, X.5.2
The basic codes it travels with
- 500 Syntax error, command unrecognized
- 501 Syntax error in parameters or arguments
- 502 Command not implemented
- 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
- 555 MAIL FROM/RCPT TO parameters not recognized or not implemented
In practice
X.5.2 is the server saying it could not parse what you sent. Common causes are missing angle brackets around addresses in MAIL FROM or RCPT TO, wrong line endings, and non-ASCII bytes in commands. Gmail uses 555 5.5.2 and 501 5.5.2 for this. A standard mail library avoids most of these problems.
What the sender should do
Check the exact commands your client sends, including line endings and argument formatting.
Would verifying the list have prevented 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.
How providers use X.5.2
Quoted from each provider’s own error reference.
- Gmail 501 5.5.2
Syntax error, cannot decode response. For more information, go to About SMTP error messages.
- Gmail 555 5.5.2
Syntax error. For more information, go to About SMTP error messages and review RFC 5321 specifications.
Common questions
- What does 5.5.2 mean?
- X.5.2 means the server could not interpret a command, because the syntax was wrong or the command is unknown.
- What should I do about 5.5.2?
- Check the exact commands your client sends, including line endings and argument formatting.
- Would an email verifier have caught 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
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
- Google Workspace Admin Help: Gmail SMTP errors and 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