Well, for the future, I'd turn on message tracking, or add a mailbox to the
DL and hide it from the GAL.

For now, you can't see anything on the Exchange server, but you may want
to ask in microsoft.public.exchange.admin to be sure.

On the Outlook side, I guess you could write a tool that would access each
users Sent Items folder and dump the list of messages and hope that no one
deletes their sent items.