If you’re seeing a lot of “451 4.7.500 Server busy. Please try again later” from Office365 this morning you’re not alone.
Microsoft are aware of the issue, and incident EX680695 says:
Current status: We’ve identified that specific IP addresses are being unexpectedly limited by our anti-spam procedures, causing inbound external email delivery to become throttled and delayed. We’re reviewing if there have been any recent changes to our anti-spam rules to understand why the IP addresses are being limited. In the meantime, we’re manually adding reported affected IP addresses to an allowed list to provide immediate relief.