X.5.5 · Mail delivery protocol
SMTP enhanced status code 5.5.5
Wrong protocol version
X.5.5 means the two servers could not agree on a protocol version. It is rare on today's internet.
| Registry code | X.5.5 |
|---|---|
| Forms in use | 5.5.5, 4.5.5 |
| Registry name | Wrong protocol version |
| 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 | Not given |
| Defined in | [RFC 3463] |
What the registry says
A protocol version mis-match existed which could not be automatically resolved by the communicating parties.
IANA SMTP Enhanced Status Codes registry, X.5.5
In practice
Protocol version mismatches are very rare today because almost every server speaks ESMTP and falls back to plain SMTP when needed. If you see X.5.5, the sending or receiving software is probably very old or very unusual, and updating it is the fix.
What the sender should do
Update or reconfigure the sending software.
Would verifying the list have prevented 5.5.5?
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 5.5.5 mean?
- X.5.5 means the two servers could not agree on a protocol version. It is rare on today's internet.
- What is the difference between 5.5.5 and 4.5.5?
- They share the subject and detail digits, so they report the same condition, "Wrong protocol version". 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 5.5.5?
- Update or reconfigure the sending software.
- Would an email verifier have caught 5.5.5?
- 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)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05