Enhanced status code Permanent

SMTP error X.5.6

Authentication exchange line too long

Updated Jul 1, 2026

The short answer

Authentication exchange line too long Common cause: AUTH response exceeds server buffer / mechanism limit

Quick fix

Use a supported authentication mechanism or update the client

5 - Permanent. The server will not try again. Fix the cause before you resend.
This code is written X.5.6 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

Use a supported authentication mechanism or update the client

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.5.5 Wrong protocol version
All SMTP error codes
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