El Capitan Outgoing Mail server problems
It seems that the solution is to delete all accounts (hoping you have no pop accounts) and recreate them, but not in Mail; go to System Preferences>Internet Acccounts to delete and recreate the accounts. When you recreate one, you have the choice to choose email account only (scroll down to Add another count (not sure of the english, I translate from french) and clic on it.
Of course, work with Mail closed.
The account you create there are created in Mail.When you finish to recreate all your accounts, reopen mail, et voilà!