Applies to: Exchange 2007 or later
If you migrate from one Exchange Server version to another, you might have different versions of Exchange servers, which is called a mixed-mode environment. While you have multiple Exchange Server versions in your environment, you must add a service account for each version of the Exchange server that you have in your environment.
Procedure
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From the navigation pane, expand Solutions > Apps, and then click Exchange Apps. 
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On the upper-right corner, click Settings. 
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On the page that appears, in the Mailbox tab, under Exchange service accounts, click Add. The Add service account dialog box appears. 
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Enter the credentials for one of the Exchange server versions in your environment, and then click Save. 
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Add an Exchange Service Account for each Exchange server version in your environment. 
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Click Save.