Strange Appointment notice

I have Exchange 2k install behind ISA server, and an Outlook 2000 environment, 
and here is a strange situation.
 
Internally, Everything works as configured.
 
Clients that also use Outlook 2000 or Outlook 2002 can also book appointments 
with our internal user as long as we give them our email address.
 
Question:
Some of these outside Outlook user will be able to book appointment with our 
internal users. When it is accepted, it updated the calendar, and if it is not 
accepted yet, it still shows in the calendar as tentative. (So they are all 
fine)
 
For the others, only the email notice comes through, and on the email there is 
no accept/tentative/decline type of acceptance options and also, there is 
nothing showing on the calendar.
 
For some, while they can send email, they will try to book appointment, but NO 
email/calendar message will come through and no error messages!
 
Any clues?
 
Many thanks
 
Eddie

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