Enhanced status code Temporary

SMTP error X.4.0

Other or undefined network / routing status

Updated Jul 1, 2026

The short answer

Other or undefined network / routing status Common cause: Unspecified network or routing problem

Quick fix

Check DNS, routing, and full server response

4 - Temporary. Safe to retry. The message is not lost and most servers retry automatically.
This code is written X.4.0 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 DNS, routing, and full server response

Related codes

X.4.1 No answer from host X.4.2 Bad connection X.4.3 Directory server failure X.4.4 Unable to route X.4.5 Mail system congestion X.4.6 Routing loop detected
All SMTP error codes
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