@tyil >The components themselves can be free software, even if they interact with proprietary systems.
The thing is, the recommended Exchange Web Services (EWS) plugin for thunderbird is proprietary software.

There is a free 3rd party EWS plugin available, but it only seems to work with an old version of Thunderbird


Evolution meanwhile has a free EWS plugin, although it sadly runs the proprietary login JavaScript (but if you can figure out how to get the required tokens without running proprietary software, you can provide those to evolution and it won't run the login JavaScript).