

I don’t have proton mail but I have non Gmail.
So, the good thing is, your emails are showing up and not disappearing into the ether like Microsoft.
Have your recipients add you to their contact list and mark you repeatedly as not spam. It may also be good to have them reply to your email.
It’ll take a bit but you’ll end up in the good side
I don’t know the specifics of being a paid Exchange customer.
From my experience, I had to switch three servers. Domain and IP reputation seem to mean a LOT to Microsoft. You can’t be on a blacklist. There are blacklists in which you can request to be removed from - Microsoft doesn’t care.
Also, without setting up all the stuff you didn’t need to do 10 years ago, Microsoft also silently sends your email into the ether: DKIM, DMARC, SPF…
It’s infuriating, especially since they run a lot of the enterprise email hosting.