Messages not retaining read/replied-to marks
Since migrating to the atmail.com cloud email service, messages in Windows Thunderbird (45.6.0) are not maintaining the status of READ or REPLIED-TO. The message appears to be correctly marked as READ and then later we come back and it is UNREAD again. The same thing happens with REPLIED-TO messages, later showing no indication that it had been replied to.
Wšykne wótegrona (1)
I think this is not a problem with TB, but with the mail server. For example, I have accounts that display the replied-to icon some of the time, and other accounts that never fail to show it. When in doubt, right-click a message without the icon, Open Message In Conversation, to see if a reply was made.