Exact response pattern
451 4.7.650 The mail server has been temporarily rate limitedTemporaryMicrosoftRate limit
What it means
Microsoft is rate limiting the sending source or tenant.
Likely causes
- Connection or recipient rate exceeded.
- Burst traffic.
Recommended response
- Lower concurrency and per-domain rate.
- Use gradual retries.
- Distribute traffic only across legitimately separate, warmed sources.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.