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.
I haven’t played it properly either. But there’s a community mod called Deus Ex Revision (It’s also on Steam). Which improves some of the graphics, and looks to include a bunch of QoL features.
https://www.moddb.com/mods/deus-ex-revision