mail addresses are a means to an end. The goal is usually not to send an email to a particular ad-dress, but to a particular person. You want to say hello to your friend Steve or send a message to the VP of marketing at Microsoft or to the head caterer for your wedding. Ideally, you could send a message to a person just by enter-ing his or her name, position, or some other descriptive attribute. If a person’s email address changes, the email sys-tem should send to the new address au-tomatically. If the person matching a description differs over time, the email system should send to the person cur-rently matching that description.More generally, you should be able to send email messages to groups of people matching a particular set of at-tributes: all Stanford department chairs, all female customers living in Detroit, or all people in your organization who speak both English and French. Today, we use mailing lists to email predefined groups of people. However, because in-finitely many ways to define a set of people exist (for example, “all people in the marketing department whose name starts with the letter ‘M’”), you gener-ally can’t rely on such lists. Instead, you must be able to address your email to static mailing lists that are the best fit to your requirements, and you must know of their existence.