envelope icons

Tried looking for the answer to this one on FAQ, but clearly it is not a question frequently asked. Tried to work it out for myself but no go there either so ...
Why are some envelope icons blue grey and some red?

 

Go into a category like novels. Look at the bottom left of the screen. You'll see an explanation there. The red envelopes indicate hot threads.

 

Thanks for your reply, must go, need to research what a hot thread is.

 

It's a thread that people post in frequently. An active thread, or a popular thread.

 

so, mallory, why's your feather so hot?

 

Simple. Mallory's a firebird!