MacBook Pro (Retina, 15-inch, Early 2013), 2.7 GHz Intel Core i7 16 GB 1600 MHz DDR3 El Capitan 10.11.3 (15D21)
For a while, mail has been doing strange things.
When I shut sown, mail can take from 20 seconds to never to quit, and I have to Force Quit the application to finish he shutdown.
Too often, it will ask for an email address and tell me the password os wrong, so I quit mail. After restarting all the email addresses work.
Many times when I send an email, after it is sent, it shows up as an archive and once I found nine emails all the same.
RichRd