Exact response pattern
451 4.4.3 Directory server failureTemporaryGenericServer
What it means
The receiver could not query its directory or recipient backend.
Likely causes
- LDAP or directory outage.
Recommended response
- Retry later.
- No sender-side suppression is warranted.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.