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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail System Overview and Concepts

TCP/IP Electronic Mail Communication Overview: Message Composition, Submission, Delivery, Receipt, Processing and Access 1 2 3 TCP/IP Electronic Mail Addresses and Addressing

The Delayed Delivery Model and Device Roles

To allow the type of communication needed for electronic mail, the entire system is designed to facilitate the delayed delivery of e-mail messages from one user to another. To see how this works, let's look again at the example communication we discussed in the previous topic, but this time, considering the roles of the different devices in the exchange (as shown in Figure 301):

• Sender's Client Host: The sender of the mail composes an electronic mail message, generally using a mail client program on his or her local machine. The mail, once composed, is not immediately sent out over the Internet; it is held in a buffer area called a spool. This allows the user to be “detached” for the entire time that a number of outgoing messages are created. When the user is done, all of the messages can be sent at once.
  
  
• Sender's Local SMTP Server: When the user's mail is ready to be sent, he or she connects to the internetwork. The messages are then communicated to the user's designated local SMTP server, normally run by the user's Internet Service Provider (ISP). The mail is sent from the client machine to the local SMTP server using SMTP. (It is possible in some cases for the sender to be working directly on a device with a local SMTP server, in which case sending is simplified.)
  
  
• Recipient's Local SMTP Server: The sender's SMTP server sends the e-mail using SMTP to the recipient's local SMTP server over the Internetwork. There, the e-mail is placed into the recipient's incoming mailbox (inbox). This is comparable to the outgoing spool that existed on the sender's client machine. It allows the recipient to accumulate mail from many sources over a period of time, and retrieve them when it is convenient.
  
  
• Recipient's Client Host: In certain cases the recipient may access his or her mailboxes directly on the local SMTP server. More often, however, a mail access and retrieval protocol, such as POP3 or IMAP, is used to read the mail from the SMTP server and display it on the recipient's local machine. There, it is displayed using an e-mail client program, similar to the one the sender used to compose the message in the first place.
  

  Figure 301: Electronic Mail (E-Mail) Communication Model

  This diagram shows the four devices that are involved in a typical e-mail communication between two users. (Yes, they are identical twins, imagine that! J) Each device consists of a number of different elements, which communicate as indicated by the black arrows. Note the inherent asymmetry, because the method used to send an e-mail from a user is not the same as that used to retrieve it from the server. The large green arrows show a typical transaction: the sender composes mail and it goes to her local e-mail spool. It is sent to the sender’s local SMTP server using SMTP, and then to the recipient’s SMTP server, where it goes into that user’s inbox. It is then retrieved, usually using a protocol such as POP or IMAP.

  
  

TCP/IP Electronic Mail Communication Overview: Message Composition, Submission, Delivery, Receipt, Processing and Access 1 2 3 TCP/IP Electronic Mail Addresses and Addressing