I have been having trouble with this specific email address. Initially emails were returned stating the email didn’t exist. Now this?! Please help!!
The issue is on the microsoft side so there really isn't much you can do. Microsoft is either blocking it on purpose or accidentally by a rule.
I tested from two of my accounts and got the same message. You originally got a user does not exist, I imagine they got rid of the email address or the domain associated with it.
You would need to contact the recipient outside of email and have them update their spam filtering but my thoughts are the email address no longer exists.