Enhanced status code Permanent

SMTP error X.5.0

Other or undefined protocol status

Updated Jul 1, 2026

The short answer

Other or undefined protocol status Common cause: Unspecified SMTP / protocol problem

Quick fix

Check the full SMTP response and client / server compatibility

5 - Permanent. The server will not try again. Fix the cause before you resend.
This code is written X.5.0 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

Check the full SMTP response and client / server compatibility

Related codes

X.5.1 Invalid command X.5.2 Syntax error X.5.3 Too many recipients X.5.4 Invalid command arguments X.5.5 Wrong protocol version X.5.6 Authentication exchange line too long
All SMTP error codes
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