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TCP/IP Electronic Mail Message Communication Model and Device and Protocol Roles
(Page 3 of 3)
Protocol Roles In Electronic Mail Communication
You may have noticed that SMTP is used for most of this communication process. In fact, if the recipient uses a machine that runs SMTP software, which is common for those using dialup UNIX shell Internet access, the process of sending e-mail uses SMTP exclusively. SMTP servers must, however, always be available on the Internet and ready to accept mail. Most people access the internetwork using devices that aren't always online or that don't run SMTP software. That is why the last step, mail access and retrieval, is usually required.
It might have been possible to define the electronic mail system so that this last step of communication was carried out using SMTP as well, which would mean the entire system used the same protocol. However, SMTP was tailored for the specific purpose of transporting and delivering e-mail, not for remote mailbox access. It made more sense to leave the function of mailbox access to dedicated, separate protocols. This not only allows these protocols to be tailored to the needs of e-mail recipients, but provides flexibility by giving users more than one option for how e-mail is retrieved. I discuss mail access protocols and methods in a separate section of the Guide, highlighting the two most common protocols: the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP).
The three protocols discussed above—SMTP, POP3 and IMAP—get the “lead billing” on the TCP/IP electronic mail stage, but they rely on two other elements to play “supporting roles”. The first is a method of addressing e-mail messages to ensure that they arrive at their destinations. The second is the set of message formats used to encode messages and control how they are delivered and used. These don't usually get as much attention as they deserve, but they do here, as I have devoted the next two sections to them.
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Key Concept: One of the critical requirements of an electronic mail system is that the sender and receiver of a message not be required to both be on the system at the time mail is sent. TCP/IP therefore uses a communication model with several devices that allow the sender and recipient to be decoupled. The sender’s client device spools mail and moves it to the sender’s local SMTP server when it is ready for transmission; the e-mail is then transmitted to the receiver’s SMTP server using SMTP. The e-mail can remain on the recipient’s server for an indefinite period of time. When the recipient is ready to read it, he or she retrieves it using one or more of a set of mail access protocols and methods, the two most popular of which are POP and IMAP.
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