anne-ology wrote:
very interesting, indeed.
And has me curiously wondering ...
are you saying that by clicking on 'forward' then changing the
'Fw'/'Fwd' in the subject line to 'Re' breaks this coding in some
e-programs ??? ;-o
For someone reading mail using a mail client which uses the in-reply-to: and references: headers, replying will keep the thread together regardless of the subject line (provided the person sending the reply adds those headers).
For someone reading mail using a mail client which uses the in-reply-to: and references: headers, forwarding an email instead of replying will break the thread, regardless of the subject line. It's a new message, usually being forwarded to a different person, not a reply to the original sender. So it's best to use "reply" when replying, and keep "forward" for forwarding.
For someone reading mail using a mail client which doesn't use those headers, but analyses the subject line, whether the messages are groups into a thread is somewhat unpredictable. It's common to add "Re:" to the beginning of the subject of a reply, so trying to exactly match the subject line will often fail. So most clients analysing the subject line probably ignore "Re:", and perhaps other languages and variations. Is "Fwd:" ignored in the same way? What about "[Fwd]"? Or "[libreoffice-users]"? It depends on how the particular client analyses the subject line. It could break apart messages which should be part of the same thread just because of some unanticipated difference in the subject line, or it could treat unrelated messages as being part of the same thread just because they have similar subjects.
I use gmail because I dislike the philosophy of the only ISP
available in this area;
(am ready to change the moment there's an alternative available
to us)
I thus had to quit using Outlook when I switched to gmail;
I don't know what Live Mail is but have noticed this option pops
up at times when I'm contacting someone through their site ...
not knowing how to use, I merely copy & paste their e-address to
an e-mail 
When I mentioned Live Mail, I was thinking of Windows Live Mail which replaced Outlook Express. That's probably what you see popping up when you click an email link on a web page. As Tom mentioned, there's also the successor to Hotmail, which is now called Outlook.com, but it may have had the "Live" name at one time...