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451 4.7.650: Microsoft rate limit on IP reputation

451 4.7.650Mail server temporarily rate limited due to IP reputationTemporary: the sender retriesFrom Microsoft 365 and Outlook.com

Example
451 4.7.650 The mail server [203.0.113.70] has been temporarily rate limited due to IP reputation.

What it means

A 4.7.650 deferral means Microsoft accepted the connection but is slowing your IP down because its reputation does not yet justify the volume. It is a 4xx code, so your server keeps the message in its queue and tries again later.

It is seen most often on Outlook.com and Hotmail destinations and on freshly assigned IPs. It is softer than the 5.7.606 ban and the S3150 block list, but if bad signals continue it can turn into either.

The sender controls the outcome: send less, send cleaner mail, and let reputation build.

Likely causes

  • A new IP started sending at full volume without a warm up period.
  • Complaint rates or spam trap hits rose, lowering the IP’s reputation.
  • Volume jumped suddenly, such as a large campaign after weeks of quiet.
  • Reverse DNS, SPF, or DKIM is missing, so Microsoft has little to trust.

How to fix it

  1. Let your MTA keep retrying; most messages deliver within hours.
  2. Reduce volume and connections to Microsoft hosts and spread sends over the day.
  3. Warm up new IPs by starting with your most engaged recipients and raising volume gradually.
  4. Sign up for SNDS and JMRP to see Microsoft’s view of your IP and receive complaint reports.
  5. Remove inactive and complaining addresses from your lists.

Checks that apply

  • Reverse DNS (PTR) A matching PTR record is a basic trust signal for a mail IP.
  • Blacklist Check Listings elsewhere are an early warning that reputation is slipping.
  • Email Deliverability Pulls authentication and reputation signals together so you can see what to improve while throttled.
  • MailFlow Monitor Watches test mail over time, so you notice when deferrals stop or turn into rejections.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

Will my mail be lost after a 4.7.650?
No, it is a temporary error. Your server retries, and if it still fails after the retry period (often several days) you get a final bounce.
How long does warming up an IP take?
Usually a few weeks of steady, wanted mail at gradually higher volume.

Microsoft 365 delist portal Outlook.com SNDS

All 191 bounce codes