X.5.4 · Mail delivery protocol
SMTP enhanced status code 5.5.4
Invalid command arguments
X.5.4 means a valid command had invalid arguments, for example an empty or malformed HELO name or an unsupported parameter.
| Registry code | X.5.4 |
|---|---|
| Forms in use | 5.5.4, 4.5.4 |
| Registry name | Invalid command arguments |
| 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 | 451, 501, 502, 503, 504, 550, 555 |
| Defined in | [RFC 3463] |
What the registry says
A valid mail transaction protocol command was issued with invalid arguments, either because the arguments were out of range or represented unrecognized features. This is useful only as a permanent error.
IANA SMTP Enhanced Status Codes registry, X.5.4
The basic codes it travels with
- 451 Requested action aborted: local error in processing
- 501 Syntax error in parameters or arguments
- 502 Command not implemented
- 503 Bad sequence of commands
- 504 Command parameter 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.5.4 is often about the HELO or EHLO name. Gmail rejects an empty or invalid argument with 501 5.5.4, and many servers reject a HELO that is not a fully qualified domain name. Set your server's hostname to a real domain that resolves, ideally the same name as its PTR record.
What the sender should do
Check the arguments your client sends, especially the HELO or EHLO hostname, which should be your server's fully qualified domain name.
Would verifying the list have prevented 5.5.4?
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.5.4
Quoted from each provider’s own error reference.
- Gmail 501 5.5.4
HELO/EHLO argument argument invalid closing connection. For more information, go to HELO/EHLO email error.
Common questions
- What does 5.5.4 mean?
- X.5.4 means a valid command had invalid arguments, for example an empty or malformed HELO name or an unsupported parameter.
- What is the difference between 5.5.4 and 4.5.4?
- They share the subject and detail digits, so they report the same condition, "Invalid command arguments". 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 5.5.4?
- Check the arguments your client sends, especially the HELO or EHLO hostname, which should be your server's fully qualified domain name.
- Would an email verifier have caught 5.5.4?
- 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