My email server changed recently from one IMAP server to another. Since the change, Thunderbird keeps marking some messages as Unread. Often these are old messages, reveived weeks or months ago and definitely have been read. The process appears to be random: I laboriously work through multiple folders, marking them as Folder Read. Only to find, next time I open Thunderbird, that there is a new set of messages and folders flagged as Unread. In Settings I have 'Automatically mark messages as read - Immediately on display' checked, which is the desired and normal behaviour. I'm stumped!