Microsoft teased some new windows icons a few months ago. This month they went into beta and now we see the first ones released to the public.
Microsoft updated the Mail and Calendar app icon for all users. This means you will get the new icon once you update the app in the Microsoft store or wait until it will be installed automatically. The release preview ring for windows insiders now got some more of these new icons and even more are on the fast ring. It would seem that before the summer all new icons are in place.