Electronic Mail in the Internet, Computer Networking

Electronic Mail in the Internet

Along with the Web, electronic mail is one of the most popular Internet applications. Just like ordinary "snail mail," email is asynchronous -- people send and read messages when it is convenient for them, without having to coordinate with other peoples' schedules. In contrast with snail mail, electronic mail is fast, easy to distribute, and inexpensive. Moreover, modern electronic mail messages can include hyperlinks, HTML formatted text, images, sound and even video. In this section we will examine the application-layer protocols that are at the heart of Internet electronic mail. But before we jump into an in-depth discussion of these protocols, let's take a bird's eye view of the Internet mail system and its key components.

Figure  A bird's eye view of the Internet e-mail system.

Figure presents a high-level view of the Internet mail system. We see from this diagram that it has three major components: user agents, mail servers, and the Simple Mail Transfer Protocol (SMTP). We now describe each of these components in the context of a sender, Alice , sending an email message to a recipient, Bob . User agents allow users to read, reply to, forward, save, and compose messages. (User agents for electronic mail are sometimes called mail readers, although we will generally avoid this term in this book.) When Alice is finished composing her message, her user

agent sends the message to her mail server, where the message is placed in the mail server's outgoing message queue. When Bob wants to read a message, his user agent obtains the message from his mailbox in his mail server. In the late 1990s, GUI (graphical user interface) user agents became popular, allowing users to view and compose multimedia messages. Currently, Eudora, Microsoft's Outlook Express, and Netscape's Messenger are among the popular GUI user agents for email. There are also many text-based email user interfaces in the public domain, including mail, pine and elm.Mail servers form the core of the e-mail infrastructure. Each recipient, such as Bob, has a mailbox located in one of the mail servers. Bob's mailbox manages and maintains the messages that have been sent to him. A typical message starts its journey in the sender's user agent, travels to the sender's mail server, and then travels to the recipient's mail server, where it is deposited in

the recipient's mailbox. When Bob wants to access the messages in his mailbox, the mail server containing the mailbox authenticates Bob (with user names and passwords). Alice's mail server must also deal with failures in Bob's mail server. If Alice's server cannot deliver mail to Bob's server, Alice's server holds the message in a message queue and attempts to transfer the message later. Reattempts are often done every 30 minutes or so; if there is no success after several days, the server

removes the message and notifies the sender (Alice) with an email message.

The Simple Mail Transfer Protocol (SMTP) is the principle application-layer protocol for Internet electronic mail. It uses the reliable data transfer service of TCP to transfer mail from the sender's mail server to the recipient's mail server. As with most application-layer protocols, SMTP has two sides: a client side which executes on the sender's mail server, and server side which executes on the recipient's mail server. Both the client and server sides of SMTP run on every mail server. When a mail server sends mail (to other mail servers), it acts as an SMTP client. When a mail server receives mail (from other mail servers) it acts as an SMTP server.