Email Knowledge Bounce codes

5.4.4 — Unable to route (host not found)

The enhanced status code 5.4.4 usually means the sender could not find a valid route to the recipient’s mail server. In practice, this often points to DNS problems: missing or broken MX records, or MX hostnames that do not resolve to usable IPs.

What 5.4.4 means

Email delivery is routed using DNS. When sending to a domain, the sender looks up its MX records. If the lookup fails, returns no usable results, or points to hostnames that cannot be resolved, the sender cannot decide where to deliver the message — and a routing failure like 5.4.4 can result.

Sometimes the domain itself exists, but its email routing is misconfigured. In other cases, the domain may be expired, parked, or the DNS zone may be incomplete. The bounce text often includes “host not found”, “domain not found”, “no MX”, or similar wording.

What it can look like

Action: failed
Status: 5.4.4
Diagnostic-Code: smtp; 550 5.4.4 Unable to route / host not found

Quick DNS checks

If you control the recipient domain, check that MX exists and that the MX hostnames resolve: 

dig +short example.com MX
dig +short mail.example.com A
dig +short mail.example.com AAAA

If there is no MX, some systems fall back to the A/AAAA record of the domain itself, but you should not rely on that behavior. A clean MX setup is the standard for reliable delivery.

Routing failures are not the same as policy failures. If you see 5.7.x, the mail reached the receiver but was rejected by policy. 5.4.4 is about not finding a route in the first place.

Related codes

5.4.0 (routing issue, general), 4.4.x (temporary routing problems), and the full list: /bouncecodes/.