MailFixIt

SMTP routing error

SMTP 5.4.4: Unable to route

A mail server could not find a valid next hop for the recipient. The server named in the bounce is where troubleshooting should begin.

Problem description

The registered response text is:

5.4.4 Unable to route
Is this temporary or permanent?

A 5.4.4 status is permanent for this delivery. Submit a new message only after the address or route is corrected.

Find the broken route

  1. Confirm the recipient address, especially the domain after @.
  2. Find the server that generated the 5.4.4 response.
  3. If it uses public DNS, check the recipient domain’s delegation, MX records, and MX host addresses.
  4. On that server, check accepted domains, connectors, transport maps, and smart hosts.
  5. Remove stale migration routes or add the intended next hop.
  6. Send a new message after the route is valid.

If you do not manage the generating or recipient system, send the full bounce to its administrator.

Frequently asked questions

Can missing MX records cause SMTP 5.4.4?

Yes, when the server uses public DNS routing. Private connectors, accepted-domain settings, transport maps, and smart hosts can cause the same status.

Will changing SPF fix this error?

No. SPF authenticates a sender; it does not create a route to the recipient.

Related email diagnostics

SMTP 5.4.3: Directory server failureDiagnose SMTP 5.4.3 when a routing directory or DNS lookup cannot resolve the next mail destination. SMTP 550 5.1.2: Bad destination system addressFix SMTP 550 5.1.2 by checking the rejected recipient address and the recipient domain's DNS and mail route. Email delivery delayed or rejected after a DNS migrationDiagnose email that stopped arriving after a DNS change. Compare public MX records with the intended provider and avoid false alarms for mail gateways.

Primary sources: IANA SMTP Enhanced Status Codes, RFC 3463: Enhanced Mail System Status Codes.