Exact response pattern
421 4.3.2 System not accepting network messagesTemporaryGenericAvailability
What it means
The destination is temporarily not accepting mail over the network.
Likely causes
- Receiver maintenance.
- Capacity or transport outage.
Recommended response
- Retry later.
- Avoid aggressive reconnect loops.
- Monitor provider recovery.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.