I've been playing with this somewhat since yesterday and I can confirm Christo's statement that NVDA does not read, for lack of a better phrase, the forwarding or reply status of a message in one's inbox under Outlook 2010.
I even tried removing the icon column to see if Outlook would stop stripping the "Fwd:" or "Re:" prefix from the message subject in its own display, and it did not. The icon presented, when that column is present, for a forwarded message is no different than any other new message.
What's even more perverse to me is that when there is a conversation (AKA thread) created by responding back-and-forth to a given message my inbox display is not even showing the subject itself for anything but the conversation itself. You only see a subject if you hover over the subject area, which would gain focus as you go through the list with a screen reader, but I can't imagine what earthly purpose is served by this and it's not been called-out as a bug, yet, and remains unexplained.
I've used Outlook 2007, but it's been a very long while since I did so, and have also installed instances of Outlook 2013 and 2016, but don't have them at hand, either.
I was always accustomed to knowing whether a message was a forward or reply based on the subject itself having the correct prefix. I could swear that this prefix was present and being read in Outlook in the past, but it's not on my Outlook 2010 instance and I have yet to identify a way to change that.
I worry a lot. . . I worry that no matter how cynical you become it's never enough to keep up.
~ Trudy, in Jane Wagner's "Search for Signs of Intelligent Life in the Universe"