Exact response pattern
451 4.4.1 No answer from hostTemporaryGenericNetwork
What it means
The next SMTP hop did not respond.
Likely causes
- Destination outage.
- Firewall.
- Bad route.
Recommended response
- Check MX resolution and connectivity.
- Retry later.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.