With the release of Exchange 2010 SP1, we introduced the /hosting mode switch – a feature which deploys Exchange using an Active Directory structure that affords complete separation between tenant organizations shared on the same underlying platform. /Hosting mode makes the need for a solution like Hosted Messaging and Collaboration largely redundant when hosting multi-tenant Exchange. /Hosting mode does not ship with any automation tools necessary for hosters to operate a service at scale, but it does address the requirements typical of a multi-tenant infrastructure (such as tenant organizations and service plans). Read more from the exchange team blog, Click here