The following article should be helpful in getting your Yahoo Plus mail working with the Mail app again.
I have been having the same problem.
Since August 3, Mail has not received any new messages from my hotmail account. Up until August 3, Mail has been performing flawlessly for years, handling three different email accounts [two of the accounts were disabled in Mail a few months ago].
I have the "Activity" window [Window>Activity] in Mail open. The activity window clearly shows that Mail is "Preparing to retrieve messages". But nothing is retrieved. And when I go to my email via web browser, it clearly shows that I have received email messages since August 3.
And the Connection Doctor in Mail [Window>Connection Doctor] clearly shows a green light for my hotmail/Outlook account.
I have no clue what is happening.
Just for giggles, I enabled my other other email accounts. Both retrieved emails flawlessly. Maybe Hotmail/Outlook has changed some of its security settings???