Wige is correct with what he said but I would like to add some additional caution. Now I'm not sure about Yahoo or MSN and how they connect or if this works with them for I never tried it, but I will say do not believe someone just because their IP address resolves. Back in the day "we" use to bounce across many proxies or bounced through Vhost setup on full shell accounts. "I did not not own the shell account". Depending on if an IP was assigned to the shell or not determined on if the address resolved. For the most part though we could be anyone we wanted to be and spoof just about any IP address. Again I'm not sure if this can be done with Yahoo/MSN or not but I know it works with IRC and Web browsing

Note they could also be bounced through a winbnc through someone else who is also running it.
Again this is all old school and it has been years since I have done or read anything on such things.

Wige is way more in the know of this stuff and he has really kept up with it "as much as I can tell from following his post" and I'm hoping he jumps back in here and lets me know if this type of spoofing still works and if it is still being used and is there any way of detecting it.