The main username may look like it is setup as an email, but if you wish to use it, you will need to add it as an email account. The instructions below will guide you through creating an email account in cPanel.
Login to your cPanel Account via http://www.yourdomain.com.au
Once at the main page, left click on the 'Email Accounts' icon.
In the fields provided, type the name of the account, the password and the disk space you would like to assign to the mailbox.
Left click on the 'Create Account' button to create the mailbox.
You have successfully created an email account!