Microsoft 4.7.500 is throttling. The message has not been rejected, it has been deferred because Microsoft is limiting how much it will take from you right now. Reduce concurrency for Microsoft destinations and let the retries run.
What it looks like in the log
451 4.7.500 Server busy. Please try again later from [x.x.x.x].
421 4.7.500 Server busy, please try again later.
Why it happens
Too many simultaneous connections to Microsoft MX hosts.
Sustained volume above what your IP reputation supports at Microsoft.
Reputation degradation showing up as tightening limits before it becomes an outright block.
What to do
Cut per-destination concurrency and connection rate for Microsoft, not for everything.
Spread delivery over a longer window rather than bursting.
Check SNDS. Persistent throttling is usually an early warning of a reputation problem, not a capacity issue on their side.