Nathaniel Borenstein writes, quoting Steven Rohall:
As one who has been involved in thread "calculation" for some time, I
think the biggest advantage of such a scheme is that we can take the
guesswork out of threads. In particular, if we define a Threads
header that contains all of the lineage information known at message
composition, then UAs don't need to calculate threads in some
arbitrary fashion at message reception; instead, the threads are
precisely known.
A lot of clients seem to put random junk into references and
in-reply-to. Why would their "threads" generation be any more
trustworthy?
--Arnt