My company hosts, among other things,mailboxes for several of its clients. The users for each client are located in their own (child)domain in a single AD forest. Currently I am building a brand new environment based on Exchange Server 2007 that is supposed to host mailboxes for these clients in one Exch. organization. One of the key demands from the business is that users from different clients are not able to "see" each other or even now about each others existance. So far we managed to build such an environment with one exception: when a user knows the SMTP address of a user ofsome other client it can see the free/busy information of this user when scheduling an appointment in OWA. He is, however, not able to resolve the SMTP address to a display name nor search for the user in the GAL. This has got to do something with the availabilty service I believe. Does someone knows way to restrict the availability service or some other solution? Thanks in advance Roland