Thank you for your comments and concerns, Randy. We are very familiar with web hosting. Maximum flexibility in the majority of scenarios is achieved when the domain is given a maximum disk space limit. In most control panels and cases the domain is billed by the allocated disk space. The domain admins then are able to set user mailbox size accordingly.
We have not encountered your particular scenario before. We will continue to monitor this community post, and if there are several up votes, then we may take the implementation under consideration.
Since this is not necessarily a problem with SmarterMail, I will also change this from a problem to an idea thread in order to facilitate community voting.