When things appear with a green at symbol in front of them then it's an indication it has been "mentioned".
It's one of those 'new features' of the forum, that allows you to mention people or spaces etc. in a post and then those people, spaces receive a notification and activity showing the mention.
How that's working from the email responses, I'm not sure as I haven't come across any myself yet.
But let's just mention you Dude in this post and your communications will get two entries, one saying I've responded to this discussion and one saying I've mentioned you in this discussion.
If I understand correctly now, the only I will see the green @ messages in the overview listing? I actually thought those responses were email because of the @ sign.
What about those other resent messages, they are also visible without logging in.
jgarry mentioned Community Feedback and Suggestions (Do Not Post Product-Related Questions Here)
Apparently what was mentioned here was the name of the community. I wonder how much sense it makes to have this as a separate notification item.
I think the point of the 'mentions' is that it can draw the attention of the person or space mentioned to the post. So if someone posts something in a space which may be sort of relevant, but may be better asked in another space, someone can just mention "You'd probably get more help from the whatever space" (linking to the relevant space), and then that mention would appear in the activity of that space, so people hanging around in there will see it and 'come to the rescue!' without a moderator or whoever moving the post to that other space. (though we still do move obvious ones that are in the wrong place).
As I understand it, it comes down to more of the cloud concept where things are just floating about in space and can exist in multiple forum spaces or people spaces just because they are mentioned (which is a bit like tagging things I guess).
How it all works with the email concept... well... I'm still learning like everyone else.