Microsoft is testing an updated Mail app with Fluent Design with Insiders

[WindowsCentral.com]

A new Mail app is now in testing with Insiders in the Skip Ahead ring that brings new Fluent Design effects to Mail and Calendar!
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.

Full story from the WindowsCentral blog...