Enhanced SMTP status code · RFC 3463 · IANA
5.5.5 Wrong protocol version
The two servers use protocol versions that do not match.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
- Subject: X.5 · Protocol
Failures of the delivery protocol: implementation errors or an unreliable connection.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026