what does the box symbol mean next to messages in inbox

[Clueless]

What does the symbol mean that looks like a box and has a little check mark,
x and ? in it?

 

[neo [mvp outlook]]

Best guess is that it sounds like different meeting notice acceptance types.
In any event, take a peek at http://www.poremsky.com/howto/icons_mail.htm
since they say a picture is worth a thousand words.