Out of the box, an End User chooses to send an email out of a Request, or otherwise using a search to look up other users, they are able to see and search all Contacts out of the MDB.
Is it possble to restrict this over data partitions?
Log in as a "Service Delivery Administrator" user (e.g. spadmin) and go to Administration > Configuration > User Default.
There you can change the User Search Scope setting from its default of "Enterprise" (where users search all users in the system) to "Business Units", which will make the search scope only users that exist in their business unit.