When you have external clients that use Microsoft Exchange ActiveSync to access their Microsoft Exchange mailbox that is published through Microsoft Forefront Unified Access Gateway (UAG) 2010 Service Pack 1 (SP1), users may be able to view other users' mailboxes.
This problem occurs because of an issue in which external Exchange ActiveSync client sessions are mapped incorrectly to existing internal connections.
To resolve this problem, install the service pack that is described in the following Microsoft Knowledge Base article:
2710791 Description of Service Pack 2 for Forefront Unified Access Gateway 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates