Codenamed Monarch, the app is based on the already available Outlook Web app available in a browser today.
The big idea is to create a "One Outlook" vision which works across PC, Mac, and the Web. Vole has Outlook Web, Outlook (Win32) for Windows, Outlook for Mac, and Mail & Calendar on Windows 10 which makes everything look a bit messy.
The project will deliver Outlook as a single product, with the same user experience and codebase whether that be on Windows or Mac. It'll also have a much smaller footprint and be accessible to all users whether they're free Outlook consumers or commercial business customers.
It will have support for things like offline storage, share targets, and notifications. We should see the Monarch client towards the end of this year and there are plans to replace the Mail & Calendar apps on Windows 10 sometime in 2022.
Vole wants to replace the legacy Win32 Outlook client eventually but that looks like it will be rather a larger project.