Does anyone know a good way to achieve replying to a message into the same IMAP folder without sending another duplicate copy over SMTP? This is to be achieved with a conventional GUI mail client, especially Thunderbird.

Goal is to have an in-folder conversation. This is possible with shared IMAP folders, with per-user Seen indices, where subscribed recipients are guaranteed to see the new messages without them arriving from internet.

Thunderbird is capable of storing a copy of a reply in the same folder as original message (Account Options), which is half way there. Just pressing Save sends the message into Drafts and that is probably an even bigger patch to try to put a "draft message" into the same folder as original.

All options, client, server or logic-wise are an acceptable answer, including programming i.e. patching/creating add-on for Thunderbird.