Enhanced SMTP status code · RFC 3463 · IANA
4.4.3 Directory server failure
The message could not be routed because a directory server (usually DNS) was unavailable.
- Class: Temporary failure
Class 4: a persistent transient failure. The message was fine, but a passing condition delayed or stopped delivery; a later attempt may succeed (RFC 3463 §3.1). The sending server retries on its own for a while. The registry says this code is only useful as a temporary failure.
- Subject: X.4 · Network and routing
Network and routing status (DNS included); under the control of the destination's or an intermediate server's administrator.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026