Mail (Yosemite): If a mail server rejects your login information

Make sure you enter the password for your email account, not your OS X user account or another account.

Make sure you enter uppercase and lowercase characters where needed.

Verify your login information in Mail by choosing Mail > Preferences, then clicking Accounts. If your account provider offers webmail, try using your account name and password to access it. If that doesn’t work (or you don’t have webmail), contact your provider.