Mozilla Foundation did a deep dive into this. And the results where abysmal. The only brands not completely horrifying where Renault/Dacia because they are European and only serve the European market so they have to follow GDPR.
The App “Nine - Mail & Calendar” used to be quite good for Exchange. I used it years ago and was quite happy. Currently it has only 3.5 stars on the Play Store so I don’t know how good it still is.
Back when I used it I checked it for privacy and considered it save. No idea about the current status though. It is not FOSS.
Mozilla Foundation did a deep dive into this. And the results where abysmal. The only brands not completely horrifying where Renault/Dacia because they are European and only serve the European market so they have to follow GDPR.