Exact response pattern
451 4.7.1 Service temporarily unavailableTemporaryApple iCloudPolicy
What it means
Apple is temporarily deferring the source or message.
Likely causes
- Rate pressure.
- Reputation or authentication concern.
- Temporary service issue.
Recommended response
- Reduce rate.
- Verify DNS and authentication.
- Retry with backoff.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.