It’s a trick. You can’t import addresses or Calendar events into the new Mail app.
BOO HISS MAILSUCKSBIGTIME.
Whoa up. There are two other apps in the Windows 10 arsenal which will work as standalone apps OR in conjunction with the Mail app..
1.. People, as in the app named ‘People’
This is how you import a contacts list into People..
2.. Calendar, as in the app named Calendar
This is how you import calendar events into Calendar
Now that you know as much as I do about the Windows 10 Mail app, you can use it to your advantage, albeit in fairly basic form. If you want to know more about the Mail app, you will have to do what I do.. keep trying the different features until such time as you know what you are doing, or at least at the point where you get predictable results..