Computer Networking: Internet Protocols in Action

Chapter 2.4. Sending and Receiving E-mail with SMTP and POP

INTRODUCTION

E-mail was one of the first "killer" applications in the Internet and after web browsing; it is probably still the most popular.

Most e-mail clients (Outlook, Eudora, Netscape mail, etc.) allow users to compose e-mail messages and place them in an outgoing mailbox, send all messages in the outgoing mailbox and retrieve new mail messages into an incoming mailbox. Most users are aware that they must be connected to the Internet to send and receive messages, but they may not know the details of the network traffic required. This exercise will help you explore these details.

The process of sending mail is quite different than the process of receiving mail; they even use different application level protocols. E-mail clients typically use Simple Mail Transfer Protocol (SMTP) to transfer outgoing mail and the Post Office Protocol (POP) for retrieving incoming mail. However, there is a good deal of variation. Internet Message Access Protocol or IMAP is another popular protocol for retrieving messages. Some e-mail clients retrieve mail through a file system shared with the mail server. Some e-mail clients can also use proprietary protocols in addition to open Internet standards when transferring mail. Some e-mail clients even send and receive e-mail using HTTP.

When sending mail, an e-mail client typically connects to a single local mail server. It sends all the outgoing mail to this server regardless of the address of ...