Sent email randomly disappears

My boss has just two emails in her sent items folder today, even though she
has sent numerous email. Only some of her email from yesterday appears in
the sent item folder. And there are no emails in the sent items folder for
10/9 through 10/12. We believe the email is being sent, it is just not
appearing in her sent folder. This happens randomly, every 3 to 4 months.
It doesn't appear in the deleted items folder either, but we can recover it.
We're using Windows XP with Office 2007; Outlook is running in cached mode.
We have Exchange 2007. Why is this happening and how can I prevent it from
happening again?