Does anyone know of a program that authenticates addresses and zip codes?

I'm looking to integrate into a VFP app some sort of methodology to validate zip codes entered by the user. The third-party app could charge you for this or not... doesn't matter. The most important part would be that it was available through the net or a .dll that would allow me to programmatically manipulate it. Furthermore, it would need to be a system that was dynamic and changed whenever the US Post Office changed or updated zip codes etc, to insure its current and accurate.

Thanks for the help.
Shawn Burke
DRI Management Systems