Enhanced SMTP status code · RFC 3463 · IANA
5.4.4 Unable to route
The next server could not be found because routing information is missing; for example, the domain's DNS returns only an SOA, with no MX or address.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1). The registry says this code can be temporary or permanent.
Also seen as:
4.4.4 - Subject: X.4 · Network and routing
Network and routing status (DNS included); under the control of the destination's or an intermediate server's administrator.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026