Each main account (each cPanel account) has one main login for e-mail that is the same as the cPanel account login. This login when used with an e-mail program must not have the @yourdomain.com portion attached to the end of the username.
Any e-mail accounts created through cPanel have a username of:
[email protected]
They must have @yourdomain.com appended to the end of them to work in an e-mail program. If your e-mail program does not accept the @ symbol then try replacing it with a % symbol or a + symbol.
In the above text yourdomain.com stands for your cPanel account's domain name.