Problem: in attempting to setup a new account in Mail, the login credentials are rejected and the setup dialog just spins forever, while the same credentials work fine in iOS.
We recently changed service providers, and my wife is unable to setup Mail to use her new account at charter.net. Opening the new account dialog, she provides her name, username and password and clicks "Sign in"; the result is a new dialog with the red error message "Unable to verify account name or password." filling in the Incoming and Outgoing Mail Server boxes (both "mobile.charter.net") and either leaving "Use Name" as "Automatic" or filling it in "UserName@charter.net" or just "UserName" makes the Sign In box active, but clicking it just causes everything to go grey and the wheel to spin.
The puzzle is that the exact same credentials work fine to log in to Charter's web mail, or to set up an account on either of two iOS devices (an iPhone and and iPad). Why don't they also work for Apple Mail on MacOS?