Assume that you have deployed Microsoft Exchange Server 2013 and Microsoft Exchange Server 2007 in a co-existence environment. After you upgrade to Cumulative Update 6 (CU6), you notice that databases unexpectedly fail over between the nodes of database availability groups.
Microsoft strongly encourages customers who are deploying Exchange Server 2013 and Exchange Server 2007 in a co-existence environment to also deploy the update that resolves this problem. Alternatively, customers can try the following methods to work around this problem:
When you migrate users to Exchange Server 2013, make sure that all mailboxes in a delegated relationship are moved together.
Make sure that users who use delegated relationships are deployed in the same version of Exchange Server.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
The problem occurs when a user whose mailbox is located in Exchange Server 2007 tries to access a delegated mailbox that is located on a server that is running Exchange Server 2013 CU6.
When the updated server that is running Exchange Server 2013 tries to authenticate the user, it uses a mechanism that is not understood by Exchange Server 2007. This causes the Exchange Server 2013 Store process to crash and fail over the database to a different server. Co-existence with Microsoft Exchange Server 2010 is not affected by this problem.