In a nutshell, Caller ID involves two key steps. One, senders of e-mail publish the IP addresses of their outgoing mail servers in DNS in an e-mail policy document. Two, the e-mail software at the receiving end of a message queries DNS for the e-mail policy and determines the "purported responsible domain" of the message. This is done by comparing the information in DNS to ensure it matches the information on the originating mail. We believe this technical solution gets at the root of the spam problem by helping to confirm legitimate senders.