Enhanced status code Temporary

SMTP error X.4.5

Mail system congestion

Updated Jul 1, 2026

The short answer

Mail system congestion Common cause: Destination system is overloaded

Quick fix

Retry later

4 - Temporary. Safe to retry. The message is not lost and most servers retry automatically.
This code is written X.4.5 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

Retry later

Related codes

X.4.0 Other or undefined network / routing status 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.4.6 Routing loop detected
All SMTP error codes
Ready when you are

Most rejections come back to sender reputation.

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