After launching Thunderbird (and fetching the latest messages) I replied to an e-mail. The e-mail was successfully sent, even the Gmail web interface has it in its "Sent" folder. Nevertheless, Thunderbird has a blinking progress bar and is still trying to move the message to the "Sent" folder. I set maximum server connection to cache to 1, but to no avail. My sent messages folder is in fact "Wysłane", not "Sent", I used the English counterpart for the sake of simplicity. I'm using Linux Mint 17.1, Thunderbird 1:31.6.0+build1-0ubuntu0.14.04.1, architecture amd64