> Is there an Outlook option to turn on In-Reply-To or References
> headers?

According to this link:
http://lists.debian.org/debian-user/1999/debian-user-199912/msg00354.html,
it doesn't do it. I found a few other posts through Gooja of people asking
this question -- none had any replies, so it seems as though Outlook is a
problem here.

Chris