Many people like you appear to think clicking reply and deleting the visible parts of the email starts a new thread. It does not as the original references etc of the email are retained You could start by clicking edit as new instead as that does convert the original email into a new one. A far more satisfactory result would be to click on the address in the old email and select compose message to from the menu that is presented. You might even want to create a mailing list of CC's for emails if you are corresponding with the same folk over and over. But what you are currently doing is never going to be satisfactory to either you or those you correspond with because they will be forever having to chase down your misplaced email that are on old unrelated topics. In Thunderbird you can manager threading in the View menu > sort by just change the sort to be unthreaded. But that is only for how '''you '''view '''your mail''' in '''your mail client'''. It does not fix the issue for everyone else.