>You are verifying that the user at that server exists.

And if it does, still means nothing. It could be one of the places used by spammers where they generate thousands of addresses daily (am: on a daily basis), and they use them perhaps for just a few days, to fool the by-address filters. So these addresses exist, in the sense that some real emails were sent from them. Doesn't imply that there ever was anyone manning that station.