Exact response pattern
421 4.3.0 Mail system fullTemporaryGenericCapacity
What it means
The receiving mail system is temporarily out of queue or processing capacity.
Likely causes
- Queue saturation.
- Disk or database pressure.
Recommended response
- Retry later.
- Lower connection pressure.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.