In a Microsoft Exchange Server 2013 and Microsoft Exchange Server 2007 coexistence environment, you cannot route Exchange ActiveSync (EAS) traffic to legacy Exchange Server 2007 mailboxes after you install Exchange Server 2013 Cumulative Update 6 (CU6).
When this problem occurs, the following entry is logged in the ActiveSync log:
Log Name: Application
Source: MSExchange Common
Date: 9/2/2014 12:16:40 PM
Event ID: 4999
Task Category: General
Watson report about to be sent for process id: 1948, with parameters: E12, c-RTL-AMD64, 15.00.0995.029, w3wp#MSExchangeSyncAppPool, M.Exchange.AirSyncHandler, M.E.A.Handler.LegacyUserAllowedToSync, System.InvalidCastException, 9619, 15.00.0995.026.
ErrorReportingEnabled: False Note This issue only occurs in Exchange Server 2013 Cumulative Update 6 with Exchange Server 2007 coexistence environment.
This problem occurs because of an issue in Exchange Server 2013 that prevents a user whose mailbox is hosted on Exchange Server 2007 from being authenticated when Exchange Server 2013 routes the call to Exchange Server 2007.
To work around this problem, route the EAS requests through an Exchange 2007 Client Access server (CAS).
To resolve this issue, install Cumulative Update 7 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.