4.4.3: DNS or directory lookup failed
The message could not be passed on because a directory server, most often DNS, did not answer; the lookup and the delivery are retried.
- Standard
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 4.4.3
- Provider
- Standard SMTP codes
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
4.4.3 Directory server failure
What it means
RFC 3463 defines X.4.3 as a failure to reach a directory server needed to forward the message, and gives an unreachable DNS server as its example. It calls the code useful only as a persistent temporary error, so 4.4.3 is the usual form and the message stays queued. RFC 7208 also told receivers to use 451 4.4.3 when an SPF check meets a temporary DNS error, a case for which RFC 7372 has since registered X.7.24. Either way, a lookup failed for a moment: on the sending side for the recipient's MX records, or on the receiving side for the sender's DNS records.
Common causes
- The sending server's DNS resolver is down, slow or misconfigured.
- The recipient domain's DNS servers are not answering.
- The receiver could not look up your SPF or other DNS records in time.
How to fix it
- 01Let the queue retry; temporary DNS failures usually clear.
- 02Check the recipient domain's MX records, and your own domain's SPF record, with a lookup tool.
- 03If you run the sending server, test its resolver and give it a second one.
- 04Domain owners: make sure your DNS is served by more than one name server.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 3463, section 3.5: Network and Routing Statusrfc-editor.org
- RFC 7208, section 8.6: Temperror (SPF)rfc-editor.org
- RFC 7372, section 3.2: SPF Failure Codesrfc-editor.org
- IANA, SMTP Enhanced Status Codes registry (Enumerated Status Codes)iana.org
Related codes
- 5.7.24SPF check error.The receiving server could not complete the SPF check for the sender's domain, either because of a temporary DNS problem or because the SPF record is broken.
- 4.4.0Network or routing problem, unspecified.Something went wrong with the network or the routing between mail servers, without a more specific explanation, and delivery is usually retried.
- 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.
- 5.4.4No route to the recipient domain.The mail system could not work out where to send the message next, because DNS gave no usable mail server for the recipient domain.
4.4.3 at other providers
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce