Copy Sent to Current is a Thunderbird extension that will allow you to easily place a copy of sent messages into the currently active folder or any other folder you choose, when you create the message. You may even choose to make no copy at all. Normally, if you want a copy of sent message, you must choose a fixed folder, either a Sent or any Other folder.

Useful, if your mails are presorted e.g. on an IMAP-Server, and you want answers to a mail be saved along with the original mail.

Activate it individually for each account in the Account Settings in the section Copies & Folders. There is a new radio button select on send.