Welldone sorting all that out in the Registry.
Sounds like the Windows Store version doesn’t associate everything right in the registry after install.
I also found even with the eM Client direct download from the website, the WEBCAL, and WEBCALS are not associated in Windows 11 and also only had option in settings defaults to open the Windows Store as well to find an app. Yet when i checked my registry they were already associated to eM Client.
eM Client was available for me to select in MAILTO GUI via the default apps in Win 11 for the direct download from the website. So must just be the Windows Store version that doesn’t seem to enable the MAILTO option in the GUI.