If I keep messages unread for a longer period of time (more days/weeks), even if I mark them later as read, the counter on app icon still counts them as unread. So I constantly have unread mark on the icon even if they are no unread messages. Only way to solve it is to delete the message, but it's almost impossible to find what message it does as it usually is old message and thus buried semewhere down and as it is no longer unread you have no idea which one it is.