SMTP enhanced status code 5.4.0

What the registry says

In practice

X.4.0 shows up when the sending side cannot say exactly what went wrong at the network level, such as a connection reset mid-transfer or an error the software could not classify. It is normally transient. If a single destination keeps producing it, test the route from your server with a manual SMTP connection to the domain's MX hosts. Because the code says so little, your own server's logs for the same delivery attempt are often more useful than the bounce. They show whether the connection was refused, timed out or reset, which points to the right fix.

What the sender should do

Let the queue retry. If it persists, check DNS and connectivity between your server and the destination.

Common questions

What does 5.4.0 mean?

X.4.0 says something went wrong with networking or routing, without saying what. It is a generic network error.

What is the difference between 5.4.0 and 4.4.0?

They share the subject and detail digits, so they report the same condition, "Other or undefined network or routing status". 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 5.4.0?

Let the queue retry. If it persists, check DNS and connectivity between your server and the destination.

Would an email verifier have caught 5.4.0?

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.