I'm seeing the same thing, Mail.app on OS X 10.4. Graham mentioned Google Groups being flaky, so maybe they changed the message ID system?
--Noah Jani Tiainen wrote: > Recently I've been experiencing threads that gets broken. > > From my MUA point of view it seems that people hit "reply" button, > clean up message and write it as a new one thus still leaving reference > information to original message in message headers. Then (at least my) > MUA shows this "new" message (remember it was reply, not new message) as > reply, not as a new one. > > Just my 2c... > >
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