So I like Protonmail a lot, but what I do not like is the "closed" ecosystem it has (you need Proton Calendar, Proton Mail, ...) - It does not integrate with ot

So I like Protonmail a lot, but what I do not like is the "closed" ecosystem it has (you need Proton Calendar, Proton Mail, ...) - It does not integrate with other protocols. I know some people that use https://mailbox.org/ https://mailbox.org/ for that reason.