Enhanced status code

SMTP error X.3.5

System incorrectly configured

Updated Jul 1, 2026

The short answer

System incorrectly configured Common cause: Receiving mail system configuration prevents delivery

Quick fix

Recipient administrator needs to fix the server

This code is written X.3.5 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

Recipient administrator needs to fix the server

Related codes

X.3.0 Other or undefined mail system status X.3.1 Mail system full X.3.2 System not accepting network messages X.3.3 System not capable of selected features X.3.4 Message too big for system X.3.6 Requested priority changed
All SMTP error codes
Ready when you are

Most rejections come back to sender reputation.

Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.

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