Assume that you have a coexistence environment that has Microsoft Exchange Server 2007 and Microsoft Exchange Server 2013 configured. An ActiveSync user in the environment has a mailbox that is hosted on Exchange Server 2007. In this situation, when the user accesses the mailbox, the ExchangeActiveSync application pool on the Microsoft Exchange Server 2013 Client Access server crashes frequently. Additionally, when the application pool is restarted, high CPU usage occurs on the Microsoft Exchange Server 2007 Client Access server, and then the system becomes unresponsive to user requests for all services on the Exchange Server 2007 host.
To resolve this issue, install the following update:
2936880 Cumulative Update 5 for Exchange Server 2013
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.