Actually, Unico is not the only one. What I've found is that Mozilla uses the In-Reply-To header whereas Outlook (at least some version) uses "Thread-Topic" and "Thread-Index".
Outlook's headers seem strange to me, as I don't know how a mailer can rebuild a thread tree with just an indentifier for the thread, but no information about the posts relationships.
*bg* my pet topic... See http://marc.theaimsgroup.com/?l=fop-dev&m=104335769809427&w=2 The "Internet Mail Service" mentioned there is MS Exchange Server, which actually is the culprit responsible for the "Thread-Topic" and "Thread-Index" headers.
Even for the Clients sending In-Reply-To and References, there are some caveats: http://bugzilla.mozilla.org/show_bug.cgi?id=191726 The Mail Client sending a bogus Referers header is actually Eudora 5.0 (>7.0 is ok).
Any ideas where this would fit in the Wiki or somewhere? Perhaps http://nagoya.apache.org/wiki/apachewiki.cgi?ToolChest/Users
J.Pietschmann
