Prevent a single user OWA access externaly, but not internally.

Here’s the scenario: Bunch of users use OWA for email. We have setup a single email address that needs to be checked by several different users. I intended to provide the credentials for the User account to the persons who will check this user accounts’ email. I would prefer if the account could be checked with OWA. However, I was asked to prevent this account from logging into OWA externally, but not internally. Is this even possible? This server is a single SBS 2003 Std. with Exchange SP2.