New
#11
OK, tested this and cannot reproduce your issue.
Booted up a Windows 10 version 1607 PC which only has one user account, an O365 / Azure AD account. Mail app was not set up, no accounts in it.
Added a Microsoft email account to Mail app, account remained a normal Azure AD account, did not switch to Microsoft account or change the sign in options in any way. Removing the Microsoft email account from mail app required only a few mouse clicks.
I don't know what happened to you, your Windows, but I've now tested this three times and Windows works as expected.
Main thing is you got it resolved :)