Inbox Specific Auditing

I am the junior admin at our firm and execs have asked me to monitor out Senior admin's access to mailboxes. I need to know how to set up Excahnge 2003 or a 3rd party tool to audit when/who the user accesses Exchange inboxes only. Not the event that is generated when a user accesses another user's calendar/inbox. Inbox accesss only if it is possible. We suspect he is reading corporate exec email.

Thanks in advance.