Exact response pattern
450 4.7.1 Unable to Relay. ATTR37TemporaryMicrosoftRelay
What it means
Microsoft did not accept the transaction because the route or recipient context was not authorized for relay. This response should be investigated as a routing or policy condition, not automatically treated as an IP block.
Likely causes
- Wrong recipient route.
- Tenant or connector policy.
- Invalid envelope or submission path.
- Temporary Microsoft routing state.
Recommended response
- Verify recipient and MX routing.
- Check envelope sender and connector path.
- Retry with backoff.
- Escalate with the complete transcript if persistent.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.