Strage very strange indeed.
If you tried the rest via powershell and it still doesn't work then.... :/
And i'm sure you have the app up to date from the store...
Do a full reset? That's such a strange problem
Have you tried another account in the mail app?
Yep, uninstall from PowerShell, restart PC, install again with PS, nope still broken, uninstall again, restart PC, install from Store, still broken, Windows app troubleshooter, sfc /scannow, dism.exe whateverthelengthycommandis.. I gave up.
Full reset isn't worth it, I have Outlook anyway, it's just out of curiosity why app that works flawlessly and I used it almost every hour, suddenly has a major bug for me.
Another account, that's a good suggestion, I'll try it tomorrow night.