This could be due to one (or more) of several factors. Please follow these instructions to determine what the exact problem is.
Go to "www.yourdomain.com/webmail" on your browser. This will bring you to our Web Based Mail. You can login and check your e-mail by entering your e-mail address and password as it was setup in cPanel.
If this works you you know your mail works and you have the right username and pass. Now try the same thing from outlook or you favorive email program. If it still does not work let us know.