Create New Account Plan. How to disable Mail Boxes

I have setup a new account plan in virtualmin. In "Allow virtual server features", i have unchecked Mail for domains, Spam filtering and virus filtering.

When i go back to the "Account Plans" list i notice that Mailboxes still shows 2. If i have disabled mail for domains, why is this the case?

How do i change this? (i dont want this plan having any mail capability)

My assumption is that this must be changed in "System Setting>Features and Plugins"? (the question is why?)