Not able to use free/busy with Exchange 2003

Hi all.

We use Exchange 2003 with Outlook 2002 and 2003 and it's calendaring
internally. I also setup the Outlook clients to use Internet free/busy so
we could invite external people to meeting. The external person uses
Outlook 2002 for POP3 and Internet free/busy. When they get my meeting
invite, it look like an email. Their Outlook doesn't recognize it as a
meeting.

Does anyone know if this is even supported? to use calendaring between
Exchange and Internet free/busy?

Thanks.
Ruben Alvarez