I think there are authoritative DNS servers which let anybody access them. The mail exchange info is usually public, though. (In the interests of smooth-running of the Internet, information such as this is available to most sources).

Anyway, I didn't find that entry in my registry, probably because the individual IP settings are set in the Dial-up Networking settings. I think it's probably best to use a public server, rather than trying to extract the addresses from the user's computer.