Please do not delete the 'Postmaster' E-Mail address. This is an address which you are given when you first sign up for hosting. This is where E-Mail ends up which is addressed to a non-existant account on your domain, and where people may send complaints about possible spamming that might have been done using your domain name. It is very important that you not only keep this account, but also check it for messages occasionally. If you are hosting others on your domain and someone is spamming, this account might have E-Mail complaints in which you may need to take action.

You may set the forwarding address for that account to any other E-Mail account you wish. Or, you can remove the forwarding address so that box is blank and then just let the box fill up. Once the box is "full" then all other E-Mail addresses to any unknown account on your domain (as well as to the 'postmaster') will be bounced back to the sender.

Please see How To Create, Delete, Change, or List E-Mail Accounts for more information including changing the forwarding address.