Mail won't accept password or quit

[JEB]

Hi all, I'm having trouble with loging into the Mail app. I'm running Sierra on a MacBook Air ( early 2015. When I open the Mail app, I get a error message "Can't connect to the account "JEB's Email" Enter the password for the user "jeb". I enter the correct password and get another error message "unable the verify account name or password. I try to quit mail but it hangs up until I need to do a Force Quit to leave the Mail app. All the Apple updates are up to date. Please help. Thank-you.

John