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What do those envelopes mean next to the thread subject. what are their meanings?

[HallsofIvy]

I had to check with other mentors myself but:

If they are grey and closed, then there are no new posts in that thread. If they are open and lit up, then there are new posts. If there is a black dot, then you have posted a response on the thread. If there is an orange dot, then it is "very busy".