Be sure you are using your full email address as your login
name@domain.com or name+domain.com.
One way you can check to see if it a server problem on our
end is to login to your webmail
http://www.yourdomain.com/webmail with your full email
address as the username i.e. username@domainame.com
and your password.
If that works then the problem is a
setting in your email client (Outlook, Entourage, Outlook
Express, etc...)