Thunderbird has chosen the worst possible email address to decide it will never mark as read, my boss's email address! Makes for very confusing work life and now I'm constantly having to hit "M" when I read her emails, which is super annoying. I've taken screenshots of the problem and my Thunderbird settings. I've seen all the previous replies about how to fix this problem, so I know what the settings are supposed to be, but the screenshot is for your verification in case somehow I'm missing something. I have checked Filters just to see if there was somehow anything set for her email, but there are no filters. All other emails from our company have the same @domain.com and none of our other emails do this.