It’s not that it’s closed, it’s more that none of the exiting email protocols support a server which can’t read your email (as it’s all encrypted). They do offer Proton Bridge which you can run locally which will handle all the decryption and local mail clients can talk to that as the would any other mail server.
I don’t know off hand if it supports calendar syncing though.
My bonus just came in today. Was planning to order one of these this week. Not sure of I’ll wait to see what’s happening at PAX. All they’ve said so far is that Steam will be there.