Microsoft wants to replace the existing desktop clients with one app built with web technologies. I understand that it's one of Microsoft's goals to make the new Monarch client feel as native to the OS as possible while remaining universal across platforms by basing the app on the Outlook website. I'm told the app will feature native OS integrations with support for things like offline storage, share targets, notifications, and more. It'll also have a much smaller footprint and be accessible to all users whether they're free Outlook consumers or commercial business customers. The project will deliver Outlook as a single product, with the same user experience and codebase whether that be on Windows or Mac.

Right now, Microsoft has a number of different Outlook clients for desktop, including Outlook Web, Outlook (Win32) for Windows, Outlook for Mac, and Mail & Calendar on Windows 10. From a report: Project Monarch is the end-goal for Microsoft's 'One Outlook' vision, which aims to build a single Outlook client that works across PC, Mac, and the Web.

This new client is codenamed Monarch and is based on the already available Outlook Web app available in a browser today. Microsoft is building a universal Outlook client for Windows and Mac that will also replace the default Mail & Calendar apps on Windows 10 when ready.