Enhanced status code Permanent

SMTP error X.5.1

Invalid command

Updated Jul 1, 2026

The short answer

Invalid command Common cause: SMTP command was unsupported or sent out of sequence

Quick fix

Fix SMTP command order or client configuration

5 - Permanent. The server will not try again. Fix the cause before you resend.
This code is written X.5.1 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

Fix SMTP command order or client configuration

Related codes

X.5.0 Other or undefined protocol status 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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