[nadams]

> but they're... proprietary

Everything you ever wanted to know about Exchange [1] (but were afraid to ask). I personally use SOGo [2] as a Exchange ActiveSync provider but I know OpenChange is a project to implement the entire Exchange stack. So I would not consider it proprietary.

[1] https://msdn.microsoft.com/en-us/library/cc307725%28v=exchg....

[2] http://www.sogo.nu/