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).
On one hand, /hosting mode solves many challenges inherent to offering this type of service. On the other hand, /hosting mode offers a reduced feature set as compared to the typical on-premises configuration. In the time since we released SP1 and /hosting mode, we have heard from customers and partners alike that many of these features are a fundamental requirement for doing business, and so to this end I announced that hosters would be supported when using the on-premises configuration from SP2 onwards (assuming their configuration meets certain design requirements). In addition, we also provided perspective on which is the right approach for hosters to take.
Please read the complete blog at http://blogs.technet.com/b/exchange/archive/2011/10/13/future-of-hosting-mode.aspx
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