X.4.3 · Network and routing
SMTP enhanced status code 4.4.3
Directory server failure
X.4.3 means the message could not be forwarded because a directory server, typically DNS, was unavailable. It is meant to be temporary.
| Registry code | X.4.3 |
|---|---|
| Forms in use | 4.4.3, 5.4.3 |
| Registry name | Directory server failure |
| Subject | 4: Network and routing |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | 451, 550 |
| Defined in | [RFC 3463] |
What the registry says
The network system was unable to forward the message, because a directory server was unavailable. This is useful only as a persistent transient error. The inability to connect to an Internet DNS server is one example of the directory server failure error.
IANA SMTP Enhanced Status Codes registry, X.4.3
The basic codes it travels with
- 451 Requested action aborted: local error in processing
- 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
In practice
X.4.3 almost always means a DNS lookup failed, either on your side or because the recipient domain's name servers did not answer. Check your server's resolver first, since a broken local resolver causes X.4.3 for every domain. If only one domain is affected, its DNS is the likely problem, and retries will succeed once it answers.
What the sender should do
Check that your server's DNS resolvers are working. The message should go through on retry once lookups succeed.
Would verifying the list have prevented 4.4.3?
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.
Common questions
- What does 4.4.3 mean?
- X.4.3 means the message could not be forwarded because a directory server, typically DNS, was unavailable. It is meant to be temporary.
- What is the difference between 4.4.3 and 5.4.3?
- They share the subject and detail digits, so they report the same condition, "Directory server failure". 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 4.4.3?
- Check that your server's DNS resolvers are working. The message should go through on retry once lookups succeed.
- Would an email verifier have caught 4.4.3?
- 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.
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05