The Multi-Tenancy and Hosting Guidance for Exchange Server 2010 SP2, February 2012 version, is ready to download.
Overview
A multi-tenant Exchange deployment is defined in this document as one where the system has been configured to host multiple and discrete organizations or business units (the tenants) that ordinarily do not share e-mail, data, users, global address lists, or any of the other commonly used objects in Exchange.
This document does not provide step-by-step instructions about how to achieve multi-tenancy with Exchange 2010 SP2, but provides information on the challenges and problems that need to be solved, and offers advice and direction to ensure the Exchange environment that you build can be supported by Microsoft.