As well as Thunderbird, I'm still using an open source email program called Virtual Access as my primary email software solely because of the re-threading abilities. My company is ONLY using Virtual Access. The problem with VA is that it stopped being developed probably 15 years years ago and its HTML support is rudimentary. VA dates back some 25+ years and may have be the inspiration for Thunderbird. In VA, it threads by Subject. You can take a message in a thread and make it the Root message for following messages or you can move a message to another thread provided it is earlier than the one you are re-threading. You can also copy or move to and re-thread the email to another folder. I've uploaded a screenshot from VA and in the upper right pane, hopefully you can see what I mean by this flexible way of threading. (I shrank the message pane to show more of the threading) I'd be willing to donate to Mozilla several thousand pounds for the sort of flexibility in Threads VA gives you.