I have an electric transmission line that is defined by several latitudes and longitudes. I can plot it with MapPoint. However, I need to determine all the addresses within .125 mile on either side of the corridor. If MapPoint can execute this, I will buy the software, which I hope will contain all the addresses in the USA.

There are a couple articles here on reverse geocoding, take a look. What you are describing is not built-in or supported functionality, but people have figured out how to do it reliably using a few different methods.