When you set up an email account the password you want to use must be entered both within the email app (Mail in this case) and at the internet web site of your email service provider. You likely mistyped something along the way. Go back and carefully confirm what passwords have been entered (1) in your email app and (2) at the web site end. All passwords must be exactly the same. If you have problems, feel free to contact the email service provider via their web site and most of them are happy to work with you to resolve problems. Good luck.