Sharing computer not securing e-mail mes

I have exchange server (MS Small Business Server 4.5) configured and Ms Outlook as a client. I setup individual user on there PC. Now if user xyz uses PC of user ‘abc’ then when user ‘xyz’ opens MS Outlook, he sees all the e-mail of user ‘abc’. The question is how come one user e-mail is seen by other user? Please help or let me know if you need more information. This is very urgent for me. Thanks in advance