What the bounce says
Exact wording as it appears in the rejection or NDR. Placeholders such as x.x.x.x and example.com stand for your own address and domain.
451 4.7.650 The mail server [x.x.x.x] has been temporarily rate limited due to IP reputation 451 4.7.651 The mail server [x.x.x.x] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.com
Microsoft is throttling the sending IP because its reputation is poor or not yet established. Delivery is slowed, not refused, and the text points at postmaster resources. The way out is to improve or build reputation, not to retry harder.
Most common causes
- The IP is new and has no sending history with Microsoft
- Recent traffic from the IP generated complaints or hit traps
- The IP belongs to a range with poor reputation
- Missing PTR or inconsistent HELO lowered the reputation score
What to verify next
- Confirm PTR, HELO and blocklist status for the IP
- Warm the IP gradually if it is new
- Review recent sending for anything abusive
- Use Microsoft’s sender support and delist portal if the IP is otherwise legitimate
What lowers reputation
Complaints from recipients, mail to spam traps, mail to many invalid addresses, sudden volume from a new IP, missing reverse DNS and inconsistent HELO all reduce the score. Shared ranges inherit the behaviour of neighbours.
A brand-new IP starts with no reputation and is throttled by design until history accumulates; that is why warm-up exists.
- Complaints, traps and invalid recipients hurt most.
- New IPs are throttled until history exists.
- PTR and HELO consistency matter.
Fix the fundamentals first
Verify PTR and HELO, check public blocklists, confirm SPF, DKIM and DMARC alignment, and review the last week of sending for anything unusual. Then reduce volume to Microsoft and ramp up gradually over days.
Use Microsoft’s tools
Enrol the IP in SNDS to see Microsoft’s view of its traffic and complaint rate, join JMRP to receive complaint reports, and use the sender support form if the IP is clean but still throttled.
Best diagnostic path
Mail Failure Doctor
Classify the complete rejection and preserve provider-specific diagnostic context.
Open analysis → Live analysisReverse DNS & HELO Auditor
Check reverse identity, forward confirmation and SMTP greeting consistency.
Open analysis → Live analysisEmail Infrastructure Digital Twin
Map the domain’s public mail infrastructure and provider relationships before remediation.
Open analysis →Known limits
- Reputation scores are not published; SNDS shows a coarse view.
- Recovery takes days of clean sending, not a single delisting.
Common questions
Is 4.7.651 different?
Same condition; 4.7.651 includes a pointer to postmaster documentation. Treat them identically.
Will waiting fix it?
Waiting without changing behaviour rarely does. Fix the causes, then ramp volume.
Does using a sending platform help?
It moves the problem to the platform’s IPs, which have established reputation, but complaints still follow your domain.
Why the exact message matters
The same status family can be triggered by different conditions, and providers frequently add diagnostic text that narrows the issue. Use the complete rejection text rather than treating the numeric code as a complete diagnosis.
Provider reference
For the provider-defined meaning and current requirements, review Microsoft Outlook.com sender troubleshooting.