Keep all emails on Exchange 5.5

Hi,
We have NT 4.0 running Exchange 5.5. Most users have Outlook 2000/XP & download their emails on to their system (on a PST file) or keep it on the Exchange server mailbox.

But then all users have options to delete their emails. The Managers want to keep a read-only copy of all user emails on the exchange server,which may or may be seen by users – BUT NOT HAVE RIGHTS TO DELETE THEIR EMAILS – any suggestions ?

Akash
Akashdeep_b@yahoo.com