Consider the following scenario:
Note This issue does not occur if mailboxes are created by using Exchange 2010 servers.
            - You have some mailboxes in a Microsoft Exchange Server 2010 environment that were moved from Exchange Server 2003 or Exchange Server 2007 to Exchange Server 2010.
 - Some users in the environment use Microsoft Outlook in cached mode to access the mailboxes.
 - The Download Headers and Then Full Items option is enabled in Outlook. Or, the Download Headers option is enabled in Outlook.
 - You apply some retention policies to the mailboxes.
 
Note This issue does not occur if mailboxes are created by using Exchange 2010 servers.