SMTP error X.0.0

Other or undefined status

Updated Jul 1, 2026

The short answer

Other or undefined status Common cause: The server only knows the general failure class

Quick fix

Read the full server response for the actual cause

This code is written X.0.0 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

Read the full server response for the actual cause

All SMTP error codes

Ready when you are

Most rejections come back to sender reputation.

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