Microsoft is about to not ship the Mail & Calendar app with Windows 10X. The sources close to Microsoft confirm that the company is about to replace with a new "universal" app, known as Project Monarch and One Outlook.
Windows 10X was expected get its production-ready build in December 2020, and Microsoft is about to start shipping the first devices in North Amrerican Spring 2021. Windows 10X build 20279 was rumored to be its RTM build.
However, something is wrong with that release, so it is currently superseded by a new build, 20280. It is now the latest build in the 10X release branch.
As you may remember, the initial release of the OS won't include classic Win32 Desktop app support, so Microsoft may probably launch the Cloud PC app streaming service around that time to bring more apps to the OS.
Mail & Calendar won't be preinstalled in Windows 10X
Latest Windows 10X builds do not include the Mail & Calendar apps which are available in desktop Windows 10 Editions. However, users can install both from the Store.
Instead of Mail & Calendar, Windows 10X will eventually include "One Outlook" - a new mail and organizer project from Microsoft.
Microsoft's goal is to create an universal Outlook client that works and looks identical everywhere. Project Monarch will utilize the existing Outlook for Web software.
The new app will work like a native Windows app, with the ability to store persistent data on your drive, and will also have everything that modern Store apps do, including notifications, sharing, and other similar options.