Since I'll assume you don't have root access to the server, and since you're indicating that the email address in question isn't sending those I'll assume that this is most likely a password compromise (if all the bouncebacks are originating from the same email account and the email account exists on your server)
In that case, you'd need to just change the password. If you're able to view the headers of the originally sent message it might be helpful as well. You may be able to find one by using the interface at WHM>>Email>>Mail Delivery Reports or if you don't have access to that the one within cPanel>>Email>>Track Delivery will work as well.