isolating users in an ISP environment

Greetings:

I have an Exchange 2003 server on a Win2003 server.  It is in an "ASP"
environment with many different internet domain names, each one being a
totally separate company.  It is important that one company does not see
anything or is aware of anything about another company.  I have followed
all the ASP guidelines afaik.

Most everything seems to be working, but when, as an Outlook 2003 client,
if I do the following, I can see all the other groups, users and contacts.
Any ideas on what I might have missed in isolating each company?

Click on "Calendar"
  Click on "Open a Shared Calendar" (it is a link under "My Calendars")
    A popup will appear.  Click in the "Name..." button
      A "Select Name" popup will appear.

In this popup, there is a pull-down menu for "Show Names from the...".
If I choose any of the pull-down items like "All Contacts", "All users",
"All Groups", then sure enough I can see all the other companies, their
users etc.!

How do I isolate these from one another so users in one company ("Group")
cannot see anyone except their own groups, members and contacts?

best regards
Mike

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