So my conclusion is: low interest for individuals, higher interest for corporate users.
We will do this development only if we can get it financed by a corporate user or similar.
For others, solutions exist, like using Microsoft clients, or adding a specific gateway to the Exchange infrastructure.
Regarding the tracker issues in MS client apps, users can probaly use Blockada or similar to cut those trackers, and we have a tracker blocker in the development pipe for 2020.
Thanks to all for the interesting feedback!
@rhunault FYI