My client deleted the sent items in outlook. We cannot find the sent items in rubbish bin and recover sent items.As a result, how can I find it in exchange server?

How were they deleted? If you have deleted item retention set for that store and you are within the retention time, then should be in 1 of 3 places:1. The deleted items folder2. The Dumpster under Deleted Items Folder3. The Dumpster under the Sent Items folder.It sounds as if you already checked all 3 places however. If you have and they arent there, then verify that deleted item retention is set for that store and user.http://technet.microsoft.com/en-us/library/bb125266.aspx