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I wrote:
> I think starting a new thread is better than posting a reply with the
> Subject line changed. The reason is the In-Reply-To and References
> headers I mentioned above. In a reply, they contain the "DNA" of the
> ancestor messages, and enable email software on the recipient's machine
> to determine where the reply belongs in the "family tree" of the thread.
> In the programs I have used, this is a function of a message's ancestry
> and is not affected by changing the subject line.

That last sentence is not always correct, though it is true for replies
I write. I changed George's subject line from "Message numbers." to
"numbered messages". Nevertheless, my Thunderbird mail program considers
it a reply, and lists it right below George's posting, indented one
level. If I had actually used the reply to launch a completely different
subject, this would be confusing.

Now here is where I was partly wrong. Thunderbird shows George's message
as the first in a new thread. But from examining his headers, I believe
it's a reply. Why? His headers include these References:




The second is Antoine's message of 2010-12-11 15:09Z, the one George
quoted. That message is still in my in-box, yet George's message is
not listed below it in the thread. Apparently, *he* can start a new
thread by changing the subject line of a reply. I cannot

Possibly this is because his headers don't include a Reply-To. I see
that even in his postings that are clearly replies. They contain only
References. My guess is that Thunderbird establishes a message's place
in a thread by the Reply-To header. If that's not present, then Subject
and Date are used. Other email software may behave differently.

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