On Tue, 13 Jul 1999 17:54:33 EDT, Valdis(_dot_)Kletnieks(_at_)vt(_dot_)edu said:
"An MUA SHOULD NOT generate a Reply-To/References header to a message
if the sender has modified the subject of the message."
On further pondering, the correct test is not "has the user modified
the Subject: tag", but rather "has the user included any cited text".
If there's cited text, it's either another entry in a thread or the
start of a new subthread. If there's no cited text, there's no
context of the thread left, and thus it's not on the thread anymore.
This of course is merely a heuristic, not an algorithm, as
every once in a while, a follow-up is made that doesn't leave any
cited material. But it's probably close enough for a "SHOULD (NOT)"
requirement.
/Valdis