There's a key on the bottom of the thread list... which I cannot seem to get to paste here.
If the envelope is open with a paper sticking out (regardless of the color), it means there are new posts. Red just means it is a "hot" thread, meaning it is popular (has received a lot of attention).
Thus, a closed envelope means there are no new posts.
If you've replied to a thread, a little arrow will appear on the envelope.
Best Regards from
Nick and the flock:
Lily & Linden - Lineolated Parakeets | Oscar & Maya - Parrotlets | Harpo - Quaker