The issue has been resolved by Microsoft. For more details please see this article at Bleeping Computer.
Update 10/11/2023 14:08 PM 9 days
Office365 appears to be deferring mail from our LAX data center for some deliveries with the message
4.7.500 Server busy. Please try again later
It does not appear to be occurring for all O365 destination recipients and is likely caused by an incomplete deployment where administrators may not have configured connection filtering rules and anti phishing rules