A good thought. I’d forgotten about Apple Mail’s domain support. IIRC, it’s limited but it exists.
Maybe time to dredge up the wiki thread I tried to make a while back:
Proton is noticeably missing from that list.
I’m not sure this is enough to make me seek a new email service (I’d certainly like to see if the company has an explanation or response before jumping ship), but it’s broken my view that Fastmail was one of the virtuous ones…