I know you were looking for Domme inout because they have the same business needs but I arrogantly think my own use may help.

Personally, I only use my Protonmail accounts for financial transactions (such as emailing a giftcard number) or the kind of discrete discussions about rates / services that occur prior to seeing someone for the furst time, but I use my old AOL account for normal emails with Dommes. That way, I get the security when I need it, but the convenience of a better overall platform for "normal" communication (not that AOL is great).

I have both apps on my phone and get notified when emails come in to either account so I don't have to check both accounts constantly (but again, your volume is probably much higher than mine).