Outlook/Exchange -Random Sent Items Missing

Running Exchange 2003 on a Windows 2003 server.. all clients are Outlook 2002 (XP).  User's Outlook is configured for mail delivery in the user's mailbox on the server... not using .pst files.  Problem is that randomly the items sent by this user NEVER show up in his sent items folder in Outlook.  We can confirm that the e-mails were delivered to recipients (both internal and external), but no indication that the mail was ever sent can be found in Outlook.  There are no filters, the checkbox is checked to put a copy of sent items in the sent items folder, I have tried deleting all the cache files and letting Outlook recreate them... but no joy.  It is possible that others may be experiencing the same problem... but no one else is as anal about checking every e-mail sent.  The only way the user can be absolutely sure to retain a record of the e-mail sent is to copy himself on every message he sends.  Experts... please rack your brains and see if you can help me out on this one..I'm stumped!