Many organizations have chosen to configure a hybrid deployment with Exchange Online to take advantage of different features such as rich mailbox moves and cross-premises calendar free/busy sharing. This includes Exchange 2003, Exchange 2007 and Exchange 2010 organizations that require a long-term hybrid configuration with Exchange Online and organizations that are using a hybrid deployment as a stepping stone to migrating fully to Exchange Online. So, at what point should these organizations decide to get rid of their on-premises Exchange servers used for the hybrid deployment? What if they have moved all of the on-premises mailboxes to Exchange Online? Is there a benefit to keeping on-premises Exchange servers? While it may seem like a no-brainer, the decision to get rid of the on-premises Exchange servers is not simple and definitely not trivial.
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Decommissioning your Exchange 2010 servers in a Hybrid Deployment – Exchange Team Blog – Site Home – TechNet Blogs