That includes Windows Desktop (win32 and UWP Intel and ARM), Outlook Web Access (OWA), and macOS Desktop,' according to a description on the One Outlook Dashboard. (See the embedded screen shot, courtesy of Outlook (or 'Monarch') is the new version of Outlook designed for large-screen experiences.
It's currently possible to see Microsoft's 'One Outlook' dashboard site by going to aka.ms/monarch. The new mail/calendar client, codenamed 'Project Monarch,' according to Windows Central, is part of the company's 'One Outlook' plan, and will be derived from the current Outlook Web app.
Microsoft is working on a single version of Outlook that will replace its current Windows 10 Mail and Calendar apps, as well as its legacy Win 32 Outlook client with a single Outlook client for Windows and Mac.
