Microsoft is testing a new Outlook app that will replace its built-in Mail and Calendar apps on Windows 10. Codenamed Monarch and “One Outlook,” the app is “a new version of Outlook designed for large-screen experiences,” according to a leaked version of the app.

While the leaked app isn’t functional without a full internal Microsoft account, the One Outlook app will replace desktop (win32 and UWP) versions of Outlook, Outlook Web Access (OWA), and the macOS desktop client. Microsoft is also hosting the full web version of this new Outlook online.

Microsoft is building a single desktop client that will be based on the web version of Outlook. The leaked app is clearly in an early state, and Microsoft even warns employees that it’s currently designed for “brave dogfooders” and doesn’t include an offline mode. Dogfood is a term used by Microsoft and other software companies to test and use early versions of their own code.

This early version has leaked, but a final replacement for the Windows 10 Mail app might not arrive until next year. Windows Central reports that this client will likely enter into a bigger preview program toward the end of 2021, with the goal to replace the Mail and Calendar apps on Windows 10 in 2022.