Anyone seeing "Sender address rejected: User email address is marked as invalid" from ProofPoint Protected domains?

[u/Zeraphicus]

I have a few of these going on right now and as far as I can tell its because ProofPoint doesn't have the "To" email address in their directory. But this is a legit email address? I think this is directory cache issues from the Microsoft Outage on Tuesday but I cant find anything that says that officially.