To make sure you have entered your username and password correctly, do the following:
Click on the "Tools" menu and choose "Accounts".
Click on the tab labeled "Mail".
Click on the tab labeled "Servers".
Account Name field should be your pop account username
Make sure that your password is typed in exactly as specified. Usernames and passwords are case sensitive.
Make sure there is a check in the box labeled "Remember Password" or "Save Password".