Active Directory contains a database of all users. The practical implementation of this database, when accessed from an Outlook client, is referred to as the Exchange GAL. When segregating Exchange, if user searches this list, you want to limit the results to recipients in the same segregated group.
This partitioning of the GAL is provided by access control lists associated with global group memberships. For each organizational unit containing a segregated group, a global group must be created. The global group membership is the users of the segregated group located in the OU. Use this group, together with other permission changes, to provide a view of Active Directory specific to the segregated group located in the organizational unit.
When partitioning the GAL, you must remove the existing default rights to see all users and objects within Active Directory. To accomplish this task, you must remove from each organizational unit the permissions assigned to the Authenticated Users group and the Everyone group, if it exists.
The host company requires permissions to see all objects within Active Directory. Add rights for the security group for each segregated group's users, thus allowing the users to see other users in their own organizational unit. Go through each organizational unit and add a security group for that segregated group. For example, for the organizational unit representing Fabrikam.com, add the Fabrikam Users security group and assign Read rights.
There are some permissions that apply only to specific clients. Your service determines which clients get support. For example, Outlook Web Access does not incorporate user permission sets when doing searches. The permissions set on OUs do not prevent search results from including other segregated group's recipients. If you are using Outlook Web Access, you must set the attribute msExchQueryBaseDN on each address list or user object to restrict the search results to include only the members of the appropriate address list.
As with all security and permissions changes, verification and testing must be performed to ensure the expected result is obtained after the change has been implemented. In this example, the Outlook client and Outlook Web Access should be tested to guarantee that each segregated group can view only their group-specific data, and that all client features operate as intended.