Vista - E-mail: threading

I've noticed a curious phenomenon when using View by conversation in e-mail view. I have rules that divert received and sent
messages on a particular topic to a single folder. Using View by conversation is an easy way to check my response to mail received.
However, if the received message has been mailed via Outlook or Exchange, it has a "Thread-index" header that seems to confuse
WLMail, and messages with similar, but not identical, thread-index headers get lumped together in a conversation even though they
have nothing else in common, and my fiendishly clever scheme collapses in chaos.
Can anyone confirm that this happens? Do any of the MIME experts know what the function of the thread-index header is (as opposed to
the message-ID)? Any other thoughts, other than this is a bug which ought to be reported?
--
Noel