Outlook 451 4.7.500 Server Busy (Reputation Throttle)

What’s actually happening

Microsoft's inbound servers are deferring, not refusing. A 4xx is a temporary failure — your mail server holds the message and retries on its normal schedule. Microsoft throws this when it's rate-limiting your sending IP, usually because you sent a burst it doesn't trust yet or the IP's reputation is thin or sliding. Small senders see it after a sudden volume jump. If it keeps escalating, you'll start seeing 550 blocks instead, and that's a different, harder problem.

Common causes

• A spike in volume from one IP that Microsoft's traffic shaping won't accept all at once
• A new or recently-warmed IP with little Outlook.com sending history
• SPF, DKIM, or DMARC not fully aligned, so the IP looks unauthenticated and gets a lower trust budget
• A jump in spam complaints or hits to Outlook.com spam traps dragging reputation down
• The IP isn't enrolled in Microsoft's JMRP / SNDS feedback programs, so reputation builds slowly and blindly

How to fix it

1. Let it retry — don't force resendsThis is a deferral. Your queue will retry over the next minutes to hours and most messages go through. Do not loop a script that hammers Microsoft on a 451; that's exactly the pattern that turns a throttle into a block.
2. Slow your send rate and add a delay between connectionsCap concurrent connections to Outlook and meter messages per minute. In Postfix, set a transport with smtp_destination_concurrency_limit and smtp_destination_rate_delay for the outlook.com / *.protection.outlook.com nexthop so you ramp instead of flood.
3. Lock down authenticationConfirm SPF passes for the sending IP, DKIM signs and verifies, and a DMARC record exists. An authenticated, aligned stream earns a bigger send budget than an anonymous one.
4. Enroll in SNDS and JMRPSign the IP up for Microsoft's Smart Network Data Services (reputation/complaint data) and the Junk Mail Reporting Program (complaint feedback loop) at sendersupport.olc.protection.outlook.com. You can't fix reputation you can't see.
5. Warm the IP gradually if it's newStart with small daily volume to your most engaged Outlook recipients and ramp over days. Opens and replies build the reputation that lifts the throttle.