Microsoft is known to be preparing to integrate Outlook across multiple platforms, something the company discussed at the Ignite conference last year. The email client is expected to move to an online platform backed by what the company calls Outlook on the web Powered Experiences (or simply OPX) and is rumored to come with the upcoming Project Monarch.
While the app is expected to be released as a preview later this year and is set for public release in 2022, Microsoft seems to be posted a screenshot in the release notes for the latest version of Office Insiders Beta Channel.
Notes contains an image consisting of two Outlook windows, with the window in the foreground showing the new Explorer feature. However, the background has a new design that includes new icons, a refreshed left window and rounded window corners. At the moment the design you see above is not something that is released for Insiders, which shows that the company probably published a mock-up for the upcoming email client and not for the final product.
In addition, the image looks more like the email client version than the web application - which makes sense since the integrations that Microsoft is preparing for Outlook are expected to be implemented on the WEB. However, there are title bar icons that make the screenshots look like a merge of the desktop email client and the web application.
It is possible that the company from Redmond is preparing a renewed design for desktops to align with the upcoming update Sun Valley of Windows 10 before proceeding with the release of OPX.