shows why a two-part e-mail address is useful. Both Keith and Conner send an e-mail to different people inside Fredsco. The text following the figure explains how the e-mail address helps in delivering the e-mail.
Note that the e-mail software on Keith's and Conner's PCs do not think about the address at all. The PCs always send their e-mails to their respective local e-mail server. In real life, when you send a paper letter, you probably give the letter to the post office by putting it in the mailbox at the end of your driveway, bringing it to the post office, or dropping it off in a mailbox set up to receive letters. In any case, you probably give your outgoing letters to the post office in pretty much the same way every time you send a letter. With e-mail, the e-mail software on your PC sends the outgoing e-mail to the same e-mail server every time as well, regardless of who the recipient is.
After Keith and Conner's e-mail servers have their respective e-mails, the servers look at the name after the @ sign, but they ignore the name before the @ sign. Their goal is to deliver the e-mail to the e-mail server at Fredsco, and the part after the @ identifies that e-mail server. It's much like how your local post office just looks at the city and state on a letter you give them, or they just look at the zip code, to figure out to which post office to send the letter. The local post office doesn't care about the name of the person on the letter or the street address; it just wants to send the letter to a post office near the recipient.
The ISP1 and ISP2 e-mail servers know how to find the e-mail server for fredsco.com because they ask another type of server called a Domain Name System (DNS) server. The DNS server tells the e-mail servers how to find the other e-mail servers. Chapter 13, "People Like Names, but Computers Like Numbers," covers the details about DNS servers. For now, just know that they can indeed find other e-mail servers easily.
Finally, after the Fredsco e-mail server gets the e-mails, it holds the e-mail, waiting on the users to check the mail. When the person who uses the sales@fredsco.com account checks his e-mail, the Fredsco e-mail server must check the entire e-mail address of all e-mail it is holding for delivery. The e-mails with username sales get delivered to that PC. Likewise, when user fred checks his e-mail, Fredsco's e-mail server delivers the mail to Fred.
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