RE: silly quesiton

So just to be clear, each user has a login on your mail server in /etc/passwd? If so, I would strongly urge you to move to using only virtual users on your mail infrastructure.

Why? Just disallow login, and that is from the perspective that a mail user should be limited mail resources.

I argue exactly the opposite. Keep as much as possible linux users. As linux has been engineered for allowing multiple user accounts, and most other virtual user providers that are used here, have not.