Recently I have been working on multi-tenancy feature for Exchange Server 2010, so I thought to write a brief about it.
With Microsoft Exchange Server 2010 SP1 built-in multi-tenant support, service providers that use Service Provider Licensing Agreement (SPLA) no longer need a solution like Hosted Messaging and Collaboration (HMC) to host multiple organizations. Multi-tenant support provides the core feature-set of Exchange Server in a manner that can be deployed to multiple customers in a single installation, and provides ease of management and flexibility of provided features to end-users.
The hosting solution available for Exchange 2010 SP1 includes most of the features and functionality available in Exchange 2010 SP1 Enterprise deployments, but also includes features and functionality that will allow you to create and manage tenant organizations.
Microsoft Exchange Server 2010 SP1 will form part of the suite of multi-tenant capable products that will replace the Hosted Messaging and Collaboration 4.5 solution.
Read more at http://technet.microsoft.com/en-us/library/ff923272.aspx?ocid=soc-n-sa-loc-services-omers