I have some experience using outlook,
Generally, when you see the symbol its because you have received new mail. (if you want go thought the options menu and turn this feature off)
The symbol will only go away if you have read all you newly received messages. Also, if you double click on the envelope icon, it will direct you to all the newly received messages.
Are you using any filers (a filter is part of the program that will redirect incoming or out going mail) for your incoming mail? You might have messages coming in that you dont see.