Monitoring mailbox access in EX5.5

Hi,

I am trying to know if someone (an Exchange Admin) is opening other peoples mAilboxes in Exchange 5.5. When we look in the Event Viewer the message “mailbox was opened by xyz while not the primary account” this could also mean that the user was looking at the calender of that person or that the mailbox was opened by the user(in exchange profile open other users mailbox).

Is there a way to be sure that the mailbox was acctually opened and not just the calendar with a meeting request or other?