You will need to create a catch all alias for webmaster@yourdomain.com

you can do so by logging into your webmail administration :

http://www.domain.com:8383

User: webmaster@domain.com
Pass: as given in the welcome message

once logged in you will see Administrative Account Options

> click on alais administration in the dropdown
> click on add alias button
> in the field alias id put: nobody
> in destination put : valid email address based on the domain you are hosting : webmaster@yourdomain.com
> click save
> logout

now any mail coming to a non existant mail address will be forwarded to webmaster@yourdomain.com