Monitor an employees mailbox

Hi all. We have been given the task of monitoring emails for a particular user by our GM due to internal reasons… We do not want that user to know we are doing this obviously. My administrator tells me that it cannot be done short of logging in as that user which is not an option as we dont have the password. We want to track past emails as well as current ones coming in. Is this possible and if so what do we have to do to either set it up or access it? We are running Exchange 2003 / Office 2003 with GFI Mail Essentials and Content Filtering so I would have thought we could have done it easily. My background is programming not admin therefore I am asking all you experts 🙂