It would be great if you could work with ProtonMail so that it will work with Airmail. It seems like more and more people want secure email.
I'd use Airmail for it if you support it...
Proton Mail now has bridge for IMAP/SMTP access, but Airmail crashes immediately when you try and use it. Needs some loving attention from the devs!
also ProtonMail is now OpenSource. Maybe there is a way to integrate it.