Don't Accept Mail to Users Mailboxes That Don't Exist! - Default Address
By default all unrouted email will be directed to your main mailbox, where it will use disk space and resources of your account. This means that someone can send to email@example.com and you'll still receive it to firstname.lastname@example.org (where inbox is your Cpanel username). This is how spammers send out mass messages as well as viruses. A common practice between worms, viruses and spammers is to generate or create a false email address they think you'll receive.
Login to your Cpanel, click on Mail, Default Address, at the bottom click Set Default Address
Select the domain in the drop down menu, I recommend repeating this for all domains listed.
In the to: field - enter
:fail: no such address here