Enhanced status code Permanent

SMTP error X.5.5

Wrong protocol version

Updated Jul 1, 2026

The short answer

Wrong protocol version Common cause: Client and server use incompatible protocol versions

Quick fix

Update or reconfigure the SMTP client / server

5 - Permanent. The server will not try again. Fix the cause before you resend.
This code is written X.5.5 because the reason is the same whether the failure is temporary (a 4 first digit) or permanent (a 5). Read the first digit of your own bounce to know which applies.

How to fix it

Update or reconfigure the SMTP client / server

Related codes

X.5.0 Other or undefined protocol status X.5.1 Invalid command X.5.2 Syntax error X.5.3 Too many recipients X.5.4 Invalid command arguments X.5.6 Authentication exchange line too long
All SMTP error codes
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