Microsoft appears to be rolling out a new update to the Mail and Calendar apps with a small group of Insiders in the Skip Ahead ring that introduces Fluent Design and a brand new layout for Insiders to test and provide feedback on.

The version in question is 8700.40315, and appears to be an A/B test scenario. We too have this version of the Mail app, but we're not seeing any of the new design changes that are in testing with some Insiders. Regardless, those that do have the new design will see many changes are currently in the works.