Outlook searches for the PR_CONVERSATION_TOPIC (Subject without any
prefixes) MAPI property.
You could use the value of PR_CONVERSATION_INDEX to search for all message
in the discussion topic, but it is not guaranteed that the recipient's reply
will preserve it.

Dmitry Streblechenko (MVP)
http://www.dimastr.com/
OutlookSpy - Outlook, CDO
and MAPI Developer Tool

"Markus" wrote in message
...
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