Sometimes sent items are stored in the sent folder of the user rather than the senders address.



If your exchange server is not up to date, with at least Exchange 2010 SP2 RU4 you must change it on the client.



For Outlook 2010 there is a fixit applied to the client if your exchange server is not supported:


http://support.microsoft.com/kb/2843677



For Exchange Server 2010 SP2 RU4 see the link below:


http://www.msexchange.org/kbase/ExchangeServerTips/ExchangeServer2010/OutlookOWA/sent-items-behavior-shared-mailboxes-update.html