I have a new windows 10 PC with the AU installed. The user account is not linked to a Microsoft account.
I decided this morning to try the Mail app instead of using OWA. (N.B. I have office 365 apps installed on the PC as well, including Outlook that is setup to use the same account, though I had been hoping to get rid of that if Mail works).
I opened up the Mail app from the windows menu which showed a blue screen and "Accounts. First things first. Let's add your accounts" with a "+ Add account" button. nothing else (i.e. no settings menus or anything else at all).
So I click the button and choose the ListItem marked "Exchange. Exchange, Office 365" and enter in my 365 email address when prompted. the click "Next". Whereupon a box shows that informs me that "Account already exists. There's already an account setup to use *** Email address is removed for privacy *** (company name)"
Particularly uninformative as how would one attempt to sign into an account that doesn't already exist?!
How can I correctly add my account?
(Note: I have seen a couple of related questions that recommend running a Powershell command with administrator rights. None of them however explained what the command actually does so I have been reluctant to apply it.)