pretty picture that computer scientists are using to identify spammers. Each dot is a purported IP address that has sent/recieved hotmail messages. Normal folk are likely to be "enmeshed" in the resulting network--with both incoming and outgoing e-mail messages tracable to their IP. Spammers are more likely to be on the outside firing messages into the larger body, with very little in the way of reply (because most people delete their spam without replying).
Probably not perfect. As noted, new hot-mail users (who have not sent or received many messages) may get tagged as spammers.