Preventing devices from emailing externally

We're using MessageLabs for spam and message filtering.  We have devices internally that we use to send documents to users internally such as scanned documents.  When the user scans the document it emails the scanned doc to the users email address.  I would like to prevent the device from sending to external email addresses and only allow it to email to internal addresses.  What would be the best way to do this using messagelabs?