I know that this question has been asked many times, but I have tried all the proposed solutions I could find on the internet with no luck. I have been tried to solve this problem for a few days. Thus, please be patient: I am still a beginner. I would like to use Thunderbird with my posteo.de email account. I can receive and read messages flawlessly, but when I send a message, Thunderbird gets stuck for minutes on "copying message to Sent folder". Then an error pops up: "Error while copying message to Sent. Retry?" Of course retrying leads to the same behavior again and again. I also get a notification "Connection to the server my.account@posteo.net timed out". I have tried the following steps so far: 1. Unchecking "Place a copy in sent folder on my.account@posteo.net" under "Copies and folders" in my account setting. After that the error do not pop up any more, but my sent messages continue not to appear in the "Sent" folder on the server. 2. Changing the connection security settings of the outgoing server in Thunderbird. 3. Trying to repair folder, or even to delete all the .msf files I could find in my thunderbird profile directory. 4. Compacting folders. The only thing that seemed to work was to delete my Thunderbird profile directory completely and set up everything again from scratch but, even after such a drastic operation, the error popped up again after sending four or five messages. I am using Thunderbird on a linux system (more precisely, a Fedora-based domain in Qubes OS) and I am confident my account setup is correct, as I let Thunderbird do all the work automatically. However, if you think it is worth, I can share the configuration details. I am also using the Enigmail and Lightning add-ons. Could they be the cause of the problem? I apologize for the lengthy post and for my poor English. Regards, Lorenzo Guerra