Envelope with a Black Dot?

[DizkneeDenna]

Searched but could not find anything about this. I published a new post and the envelope on the left is closed with a black dot on it. It is not one of the icons explained at the bottom of the page. What does it mean?

Thanks for any and all help!

 

[LisaTx]

There will be a black dot on any thread you have participated in.