X.5.0 · Mail delivery protocol
SMTP enhanced status code 5.5.0
Other or undefined protocol status
X.5.0 says something was wrong with the SMTP protocol exchange, without a more specific code. It usually points at the sending software.
| Registry code | X.5.0 |
|---|---|
| Forms in use | 5.5.0, 2.5.0, 4.5.0 |
| Registry name | Other or undefined protocol status |
| Subject | 5: Mail delivery protocol |
| Permanent or temporary | 2.x.x success; 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 220, 250, 251, 252, 253, 451, 452, 454, 458, 459, 501, 502, 503, 554 |
| Defined in | [RFC 3463] |
What the registry says
Something was wrong with the protocol necessary to deliver the message to the next hop and the problem cannot be well expressed with any of the other provided detail codes.
IANA SMTP Enhanced Status Codes registry, X.5.0
The basic codes it travels with
- 220 <domain> Service ready
- 250 Requested mail action okay, completed
- 251 User not local; will forward to <forward-path>
- 252 Cannot VRFY user, but will accept message and attempt delivery
- 451 Requested action aborted: local error in processing
- 452 Requested action not taken: insufficient system storage
- 454 Temporary authentication failure
- 501 Syntax error in parameters or arguments
- 502 Command not implemented
- 503 Bad sequence of commands
- 554 Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here")
In practice
X.5.0 is a generic protocol complaint. Gmail sends 451 4.5.0 for an SMTP protocol violation. It usually comes from custom scripts or unusual mail libraries rather than mainstream sending services. Capture the SMTP conversation with a verbose client to see which command the server objected to.
What the sender should do
Check the sending software or library for protocol errors, and read the text after the code.
Would verifying the list have prevented 5.5.0?
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.5.0
Quoted from each provider’s own error reference.
- Gmail 451 4.5.0
SMTP protocol violation. For more information, go to About SMTP error messages and review RFC 5321 specifications.
- Microsoft 365 5.5.0
550 5.5.0 Requested action not taken: mailbox unavailable
Common questions
- What does 5.5.0 mean?
- X.5.0 says something was wrong with the SMTP protocol exchange, without a more specific code. It usually points at the sending software.
- What is the difference between 5.5.0 and 2.5.0 and 4.5.0?
- They share the subject and detail digits, so they report the same condition, "Other or undefined protocol status". The first digit is the class: 2.x.x success; 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 5.5.0?
- Check the sending software or library for protocol errors, and read the text after the code.
- Would an email verifier have caught 5.5.0?
- 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)
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (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