The mail you show it threaded by conversation. It is easily changed by going to the view menu and changing the sort by to not be threaded, but you see these messages like that because your correspondents are apparently not capable of starting a new email to write, they edit an old one and hence continue the original conversation. This is a practice that should be discouraged as it leads all sorts of paper trails that have no purpose.

Matt, no wonder you are a Top 10 Contributor! MANY thanks for the very quick response and excellent advice. I was initially confused on noting that the View/Sort by/ drop-down already specified "Unthreaded". But that referred to my Inbox. The Outbox, conversely, was indeed "Threaded". This I promptly changed and the problem that has tormented me for so long is gone. My heartfelt gratitude. On the subject of TB's 'threading', I wonder if this quirky but clever e-mailer could ´thread´ in a different way. For my conservation work in different countries I rely heavily on e-mail in several languages. Might the programme's spell-checker, with the addition of a little more code, be allowed to remember the language previously used with a recipient, changing it accordingly in future messages to that person? As for the shortcomings of my correspondents, and they may be many but ANY reply is in most cases welcome, I wonder if I am guilty of the same practice you would discourage. For in my answers I do not start a new message but simply select Reply, usually retain the same Subject and, almost invariably, insert, dialogue-style, my comments in the text received. Once more, sincere thanks for your invaluable help.

Threading will be forever contentious. Right down to folk wanting like subject threaded. Other want to "manually" remove individual mails from the thread. It is one of those please most of the people most of the time things. That is the best anyone will ever do.

Then there is the conversations addon which takes things to the google level and displays everything in a google gmail type view. https://addons.thunderbird.net/en-US/thunderbird/addon/gmail-conversation-view/

As for the remembering of language, that sounds like a good idea to me. As the address book, which would be probably a great place for such information is currently being redeveloped this might be a good time to request such an enhancement. So I have filed a request. Https://bugzilla.mozilla.orgshow_bug.cgi?id=1733738

Clicking reply continues a thread and is to be encouraged. The problem chilkdren are basically those that go back to the last correspondence with the person and then click reply and edit the subject and write a new body. The email still have it's original references so it is a continuation of the thread, but it is actually a new topic created through ignorance as part of the original thread.

Thunderbird offers an edit as new option which actually does not preserve the original references for when you want the same to and CC BCC etc. Or you can click on the email address in the header and select compose mail to. Some folks just don't do address books or anything electronic well and have no desire to actually become competent. Most I find are academics or folk who can use the term Doctor or Mister who think it is someone else problem. They are just to busy for such trivialities. I would beg to differ. But usually these folk are the ones put in charge so everyone has to cope with less than acceptable performance of whatever they think is below them.

Thank you, Matt, for the further "threading" thoughts. I also appreciate the positive response to the language idea (although the link provided "can't be reached"). Tying language to e-addresses makes good sense and I hope your request will be implemented.

Happy, also, to read: "Clicking reply continues a thread and is to be encouraged." And I sympathise with your lament about those who don't use this potentially invaluable medium properly.

Since you have been so generous with your time and obviously care about making Thunderbird the best e-mailer possible, you tempt me to make another suggestion. To its credit, TB permits editing of inserted images which I have wanted to use to reduce (mainly jpgs) down to a few hundred kB. But the custom size proportional reduction is thoroughly erratic. Trying to halve a 3000 x 2000 pixel image, say, by lowering the first figure to 1500 never produces even a 40-60% result. Ideally, TB would simply request desired output size and respond accordingly. But this may be an unreasonable request. You cured the big problem and I'm delighted.