Icons to the left of the Thread titles

What do the various icons to the left of the thread titles mean? I understand the padlocked one, but what is the difference between the closed envelope and the open envelope with a little piece of paper sticking out?

Or is there some sort of key to these icons that I am missing?

thanks!

Yes, down at the bottom of the page, just underneath your display options.

open envelope = new posts
closed envelope = no new posts
padlock = thread is closed