Generic SMTP routing error
Fix SMTP 5.4.3 directory server failure
5.4.3 means a directory system needed for routing failed. The directory may be DNS, an accepted-domain table, a recipient directory, or another private routing service, so the full response and generating host determine ownership.
A generic response can read:
550 5.4.3 Directory server failure
A 5.x.x response is permanent for this delivery attempt. If the underlying outage was temporary, a newly submitted message may work after the directory recovers.
Find the failing directory
- Public DNS cannot resolve the destination or routing target.
- A private accepted-domain or recipient directory is unavailable.
- Directory synchronization left an incomplete routing object.
- A gateway references a retired connector or route.
Resolution sequence
- Identify the server that generated the DSN.
- Read its logs for the exact lookup name and directory source.
- Test public DNS only when it is the named dependency.
- Repair the private route, synchronization, or DNS delegation owned by that system.
- Submit a fresh message after confirming recovery.
Providers may use the same enhanced code differently; prefer the provider-specific message when available.
Frequently asked questions
What does SMTP 5.4.3 mean?
A directory system used for mail routing could not complete the lookup needed to deliver the message.
Is this always public DNS?
No. The directory can be public DNS or a private routing, tenant, or recipient directory.
Who should fix it?
The administrator of the server that generated the response must identify which directory lookup failed.
Related email diagnostics
Primary sources: IANA SMTP Enhanced Status Codes registry and RFC 3463.