550 5.4.1 Recipient address rejected. Access denied can mean a few things:
https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange-online/fix-error-code-550-5-4-1-in-exchange-online
I would walk through this doc and if you cant fix it, open a ticket with support. There is a link in the doc on how to do that