Exact response pattern
451 4.5.3 Too many recipientsTemporaryGenericRate limit
What it means
The transaction contains more recipients than the server allows.
Likely causes
- Large RCPT batch.
Recommended response
- Reduce recipients per transaction.
- Use smaller batches.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.