Blue Color unread mails disappears after restarting

I am having a lot of folders and with the sendfilter all the mails ( of my clients do go to the specific folders immediatly.
As i am receiving a lot of mails on a day, it's so nice i can see which mails are completely new, because they are BOLD+BLUE COLORED.
But sometimes there's a problem and the pc has to be restarted, or Thunderbird Portable has to be restarted. After restart the mails which were BOLD+BLUE COLORED are not BLUE COLORED anylonger, only BOLD ( and BLACK ). So i cannot recognize anylonger which mails are really new. I've searched very often for an app to solve this problem, but i could find one that was doing what i need.

Does anybody know a solution for it or a good app?