Related to windows 2000 exchange server

I have installed a windows 2000 enterprise exchange server on my network. A user name john on my network uses outlook express and windows 2000 microsoft outlook to access his mails.

Yesterday he showed me a problem. The problem goes like this. If a mail arrives he can open microsoft outlook and open the mail and read it comfortably. However when he opens outlook express he can open the mail and read it.

The problem arises if for the second time he opens microsft outlook for the second time he does not see the same mail he had opened previously. what is the problem. plz inform me the solution for the problem to my e-mail address subhash_sun@rediffmail.com
Thank you