Exact response pattern
421 4.4.5 Network congestionTemporaryGenericNetwork
What it means
Temporary network congestion prevented completion.
Likely causes
- Packet loss.
- Congested route.
Recommended response
- Retry with backoff.
- Check network telemetry if widespread.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.