Exact response pattern
451 4.7.1 Greylisted, please try again laterTemporaryGenericPolicy
What it means
The server intentionally defers an unfamiliar sender tuple and expects a later retry.
Likely causes
- Greylisting.
Recommended response
- Keep the message queued.
- Retry after the advertised or standard delay.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.