Exact response pattern
451 4.3.2 System not accepting network messagesTemporaryGenericAvailability
What it means
The receiving system temporarily cannot accept SMTP traffic.
Likely causes
- Maintenance.
- Service failure.
Recommended response
- Retry later with normal queue behavior.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.