If your are using Exchange 5.5, Unity definitely should only be servicing users for the Exchange site in which it is installed. If the utility shows users from other sites it is a defect. They should not be imported into Unity.
The network connectivity between sites is not reliable enough to support Unity servicing multiple Exchange sites. In 5.5, servers within the same site generally have good network connections, while the connection between sites could be considerably slower and less reliable. Unity has to connect to a server in the same site as the recipient, so it would cause us a lot of grief and performance problems trying to connect over unreliable network connections.
