Currently, however, the addon is useless with a Gmail IMAP account.
Indeed, as explained by a Google engineer in this post http://lifehacker.com/314574/turn-thunderbird-into-the-ultimate-gmail-imap-client the Google IMAP engine:
1/ ALWAYS copies sent mail to the [Gmail]/Sent IMAP folder, whatever the settings in the email client.
2/ keeps a SINGLE copy of each email (even it is "copied" to other folders), with the following result: if you copy a sent email to some target folder, and further delete this email from the [Gmail]/Sent folder and empty the trash, then this email will ALSO be deleted from the target folder!
Thus the only way out seems to MOVE the email from the [Gmail]/Sent folder to the target folder, AFTER it has been sent.
Any chance you implement this in a near future?
Thanks in advance NL
This review is for a previous version of the add-on (1.1.2).