Firstly I got this error when I first opened Thunderbird 18.104.22.168 and clicked a sent mail: I got a different error when I clicked on a received mail. After closing and opening Thunderbird I cannot reproduce the problem, I'm going to guess it's because ESET doesn't support IMAP, which brings me to the question: Why doesn't ESET support IMAP? EDIT: I can reproduce the problem every time I get a new email. EDIT2: Seems like the error is always the same The current command did not succeed. The mail server responded: expected end of data instead of """. The "" is always different.