SMTP enhanced status code 5.3.5

What the registry says

In practice

X.3.5 points at the receiving system's own configuration, for example a domain listed in DNS but not set up on the mail server, or a server that rejects mail for a domain it should accept. Because only the receiving side can fix it, the practical step is to tell the recipient through another channel, with the full bounce text so their administrator can find the problem. Including the receiving server's name from the bounce, the time and the full reply line makes it much easier for the recipient's administrator to locate the fault in their logs. Retrying unchanged will keep failing until they fix it.

What the sender should do

Tell the recipient through another channel; their administrator has to fix the configuration.

Common questions

What does 5.3.5 mean?

X.3.5 means the receiving system is not configured in a way that lets it accept this message. The fault is on the receiving side.

What is the difference between 5.3.5 and 4.3.5?

They share the subject and detail digits, so they report the same condition, "System incorrectly configured". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.

What should I do about 5.3.5?

Tell the recipient through another channel; their administrator has to fix the configuration.

Would an email verifier have caught 5.3.5?

No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.