New Outlook App For Windows and Mac 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. This new client is codenamed Monarch and is based on the already available Outlook Web app available in a browser today. 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. 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. Microsoft wants to replace the existing desktop clients with one app built with web technologies. 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.