When a user syncs a mailbox by using an iOS 6.1 or 6.1.1-based device, Microsoft Exchange Server 2010 Client Access server (CAS) and Mailbox (MBX) server resources are consumed, log growth becomes excessive, excessive growth in Recoverable Items may occur, and memory and CPU use may increase significantly.  Server performance is affected.

http://support.microsoft.com/kb/2814847