I'm a big fan of Windows Mail & Calendars due to its simplicity. Personally, I find it more intuitive for daily use cases compared to Outlook desktop and the new WebView2 powered Outlook version. Seeing Microsoft deprecating it dragged me into starting to work on Wino a couple of years ago. Wino's main motivation is to bring all the existing functionality from Mail & Calendars over time without changing the user experience that millions have loved since the Windows 8 days in Mail & Calendars
Stable releases will always be distributed on Microsoft Store. However, beta releases will be distributed in GitHub Releases. Please keep in mind that beta releases might not be for daily use, only for testing purposes and recommended for experienced users or developers. Beta releases are also managed manually. Therefore, code in the repository might be ahead of the released Beta version at the moment. Make sure to compare versions before tryout out the Beta version.
These releases are distributed as side-loaded packages. To install them, download the .msixbundle file in GitHub releases and follow the steps explained here.