451 4.7.500: Microsoft "Server busy" throttling
451 4.7.500Server busy, please try again laterTemporary: the sender retriesFrom Microsoft 365 and Outlook.com
451 4.7.500 Server busy. Please try again later from [203.0.113.80].
What it means
Despite the wording, "Server busy" in the 4.7.500 to 4.7.699 range is not about Microsoft’s capacity. Exchange Online noticed suspicious activity from your IP and is temporarily restricting it while it evaluates further. The text usually reads like "Server busy. Please try again later from [IP]", often followed by an AS reference number.
It is a temporary deferral, so the message stays in your queue. 4.7.650 is a separate, named case about IP reputation; this range is the broader evaluation throttle. If the activity continues, the IP may move to a permanent block such as 5.7.606.
Senders act here. Recipients cannot switch the throttle off for you.
Likely causes
- A new IP or IPv6 range began sending without history.
- A sudden volume increase or many parallel connections to Microsoft hosts.
- Mail content or patterns that resemble spam campaigns.
- Shared IP neighbours behaving badly.
How to fix it
- Keep retrying with normal back off; do not resend copies, which adds volume.
- Lower concurrency and per connection message counts toward Microsoft MX hosts.
- Make sure SPF, DKIM, DMARC, and PTR are correct so the IP looks like a proper sender.
- If deferrals continue for days, send from an established IP or relay and register the IP in SNDS.
- If mail eventually bounces with this code, Microsoft 365 customers can open a support case quoting the full text.
Checks that apply
- Microsoft Compliance Shows whether your domain meets Microsoft sender requirements, which affects how quickly throttling eases.
- Reverse DNS (PTR) Verifies reverse DNS for the deferred IP.
- Blacklist Check Reveals listings that suggest the suspicious activity is real.
FAQ
- Is Microsoft really overloaded?
- No. The phrase is a cover for a reputation based throttle on your IP.
- What does the AS number in the bounce mean?
- It is an internal reference for the rule that matched. Quote it to Microsoft support, but it does not change what you need to fix.