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.
From the navigation pane, expand Solutions > Apps, and then click Exchange Apps.
On the upper-right corner, click Settings.
On the page that appears, in the Mailbox tab, under Exchange service accounts, click Add.
The Add service account dialog box appears.
Enter the credentials for one of the Exchange server versions in your environment, and then click Save.
Add an Exchange Service Account for each Exchange server version in your environment.