Envelop Icon Colors

[Mother Cake]

Hi Folks,

I'm a newbie here, and I'm trying to figure out what is up with the envelop icons to the left of each thread. I got most of the distinctions, but I can't figure out why some are blue and others are red. Can someone help me out here? Thanks!

[JessicaS]

Hello!

There is a legend for that at the bottom of the forums but here it is.

[Mother Cake]

Thanks, I guess I never scrolled to the bottom. What makes a thread hot? total number of posts?