outlook 200 with microsoft exchange

I have a question.We are trying to set up an echange server that will allow our user’s to utilize Outlook 2000.We are using citrix metaframe to authenticate to the server.Our Problem is when we try to prevent a user from entering a Known user name and then logs on to the server with HIs/her own Password and user name they can get the email from the other person.Any ideas that will prevent the user from being able to access emails that are not their own?