If a mail server rejects your login information on Mac
If a mail server doesn’t accept your user name and password, try these suggestions.
In the Mail app on your Mac, do the following:
Make sure you enter the password for your email account, not the login password for your Mac or the password for another account.
Make sure you enter uppercase and lowercase characters where needed.
Verify your login information. Choose Mail > Preferences, then click Accounts.
If your email account provider offers webmail, you can also try using your account name and password to access it. If that doesn’t work (or you don’t have webmail), contact your provider.