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.
Organizations that have configured a hybrid deployment for mailbox management and hybrid feature support have also configured Office 365 Active Directory synchronization (DirSync) for user and identity management. For organizations intending on keeping DirSync in place and continuing to manage user accounts from the on-premises organization, we recommend not removing the last Exchange 2010 server from the on-premises organization. If the last Exchange server is removed, you cannot make changes to the mailbox object in Exchange Online because the source of authority is defined as on-premises. The source of authority refers to the location where Active Directory directory service objects, such as users and groups, are mastered (an original source that defines copies of an object) in a hybrid deployment. If you needed to edit most mailbox settings, you would have to be sure the Active Directory schema was extended on-premises and use unsupported tools such as Active Directory Service Interfaces Editor (ADSI Edit) for common administrative tasks. For example, adding a proxy address or putting a mailbox on litigation hold when there isn’t an Exchange Management Console (EMC) or Exchange Management Shell (Shell) on-premises becomes difficult and these simple (and other more complex) tasks cannot be done in a supported way.
Read the complete blog at http://blogs.technet.com/b/exchange/archive/2012/12/05/decommissioning-your-exchange-2010-servers-in-a-hybrid-deployment.aspx
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