Exact response pattern
421 4.4.1 No answer from hostTemporaryGenericNetwork
What it means
A required SMTP host did not answer in time.
Likely causes
- Remote outage.
- Firewall or route issue.
Recommended response
- Retry later.
- Check DNS and network reachability.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.