We discovered an issue with the latest version of Windows Mail and Calendar: 16005.13228.41011.0
Whether connecting to a Microsoft Exchange account or a SmarterMail account using EAS, it's not possible to create calendar appointments or events.
Everything else works just fine: it's just new calendar appointments that are a problem. Previous versions of Windows Mail (e.g., 16005.13228.41006.0) appear to work just fine.
If you're unsure what version of Windows Mail you're using, simply open it and click on the gears icon at the bottom of the navigation menu;
Then, click on About:
At the top of the About section, you'll see your version:
We are keeping an eye on this issue and will let you know when we've confirmed it's been resolved by Microsoft.