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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 Addresses and Addressing

TCP/IP Historical and Special Electronic Mail Addressing 1 2 TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME

Electronic mail is analogous to regular mail, but superior to it due to two main advantages of digital and electronic communication. One of these is speed, which is why modern Internet users have come up with the slang term “snail mail” to refer to the regular postal service. J But the other is also essential: flexibility. E-mail allows you to send messages easily in ways that would be cumbersome with regular mail. And one of the ways this flexibility can be seen is in addressing.

The first way that e-mail addressing is flexible is that most e-mail clients support advanced features that allow users to specify the identity of recipients in convenient ways. While TCP/IP addressing really is fairly straight-forward, remembering the addresses of everyone you know is difficult. In the real world we use address books to help us remember addresses. With e-mail, we can do the same, by allowing e-mail software to associate a name with an e-mail address. This is usually done in one of two ways.

In “old-fashioned” text-based e-mail such as that used on many UNIX systems, name and address association is performed using aliases. These are short forms for e-mail addresses that save typing. For example, my wife's name is Robyn and I often send her e-mail, but I'm too lazy to type in her address all the time. So I have defined an alias for her in my e-mail program called simply “r”. I enter the mail command and specify the alias “r” as the intended recipient, and it expands it for me. (Yeah, I could have used “robyn” but hey, I'm really lazy. J)

In modern graphical e-mail systems, aliases aren’t used. Instead, an electronic address book is usually implemented, which is of course the equivalent of the paper address book. The difference is that there is no manual copying; you just choose the name from the list using your mouse; nothing could be simpler unless the system could somehow read your mind.

Another advantage of electronic mail addressing is that it allows the easy specification of multiple recipients. With paper mail, sending a message to ten people means you need ten copies of the message, ten envelopes and also—ten stamps. With e-mail, you just list the recipient addresses separated by a comma in the recipient list:

<user1@domain1>,<user2@domain2>,<user3@domain3>…

A separate copy is mailed to each recipient, easy as can be. Of course, aliases and/or address books can be used to specify each recipient here as well, making this even simpler.

Since e-mail makes it so easy for one person to send information to a set of others, so-called one-to-many messaging, it was also one of the first ways in which electronic group communication was implemented. Prior to e-mail, sharing information in a group setting either required a face-to-face meeting, or a telephone conference call. In both cases, all parties have to be present simultaneously, and there is a cost involved, especially when the parties are geographically distant.

With e-mail, a group of individuals can share information without needing to meet or even be available at the same time. Suppose there is a group with four individuals: persons A, B, C and D. Person A has a proposal that he wants discussed. He sends it to B, C and D. Each recipient will read it at a time convenient for him or her. Each can then reply back to the group. For example, D might have a comment on the proposal, so she just sends it to A, B and C. Most e-mail clients include a group reply feature for this purpose.

TCP/IP Historical and Special Electronic Mail Addressing 1 2 TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME