When I send an email thru Outlook using my application, I would like to tag
the email with a unique number. I would like this number to be returned in
any reply.

I know I could use the subject line or body for this number and it should
get back in a reply, but I would like to use a property that is not visible
to the user and not able to be modified by them.

When you run Outlook it has an option to list all related emails. I would
not think it would use a subject line that would not necessarily be unique to
one thread, but rather must use some code not visible to the user that all
emails and replys must share. Hope someone here has an idea on what that is
and if it is available to the programmer.

Many thanks for any ideas on this,
Mark