X.5.1 · Mail delivery protocol
SMTP enhanced status code 5.5.1
Invalid command
X.5.1 means your client sent an SMTP command that was out of sequence or unsupported, for example RCPT before MAIL or DATA before any accepted recipient.
| Registry code | X.5.1 |
|---|---|
| Forms in use | 5.5.1, 4.5.1 |
| Registry name | Invalid command |
| Subject | 5: Mail delivery protocol |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 430, 500, 501, 503, 530, 550, 554, 555 |
| Defined in | [RFC 3463] |
What the registry says
A mail transaction protocol command was issued which was either out of sequence or unsupported. This is useful only as a permanent error.
IANA SMTP Enhanced Status Codes registry, X.5.1
The basic codes it travels with
- 500 Syntax error, command unrecognized
- 501 Syntax error in parameters or arguments
- 503 Bad sequence of commands
- 530 Authentication required
- 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
- 554 Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here")
- 555 MAIL FROM/RCPT TO parameters not recognized or not implemented
In practice
X.5.1 replies usually come from homemade scripts and misconfigured devices. Gmail returns 503 5.5.1 for RCPT before MAIL, DATA without an accepted recipient, BDAT without data, and for skipping EHLO. It also returns 502 5.5.1 for commands it does not implement. These are client bugs and will happen on every send until the client is fixed.
What the sender should do
Fix the sending software or script so it sends commands in the order RFC 5321 requires. The recipient address is not the issue.
Would verifying the list have prevented 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.
How providers use X.5.1
Quoted from each provider’s own error reference.
- Gmail 503 5.5.1
Bad sequence of commands. For more information, go to About SMTP error messages and review RFC 5321 specifications.
- Gmail 502 5.5.1
Unimplemented command. For more information, go to About SMTP error messages.
Common questions
- What does 5.5.1 mean?
- X.5.1 means your client sent an SMTP command that was out of sequence or unsupported, for example RCPT before MAIL or DATA before any accepted recipient.
- What is the difference between 5.5.1 and 4.5.1?
- They share the subject and detail digits, so they report the same condition, "Invalid command". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.
- What should I do about 5.5.1?
- Fix the sending software or script so it sends commands in the order RFC 5321 requires. The recipient address is not the issue.
- Would an email verifier have caught 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.
Full guides for this code
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)
- RFC 4954: SMTP Service Extension for Authentication (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