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 Electronic Mail Addresses and Addressing 1 2 3 TCP/IP Historical and Special Electronic Mail Addressing

The Special Requirements of E-Mail Addresses

Having e-mail addressing refer to the user's local SMTP server provides a great deal of flexibility compared to having addresses mention a specific client computer. But it doesn't provide enough flexibility to handle various situations:

• An organization may want to use “generic” addresses that do not specify the name of the SMTP server to handle mail. The reason is that this requires people sending them mail to know the name of that server. For example, if someone knew my real name and that I was at MIT, it would be easier for them to remember my old e-mail address as “cmk@mit.edu” than “cmk@athena.mit.edu”.
  
  
• An administrator may change which machines handle mail over a period of time. This would mean all the users's e-mail addresses would have to change— and most of us know what a pain that is. For example, if I moved from the “athena” machine to the “jabberwocky” machine, my old address would have needed to change to “cmk@jabberwocky.mit.edu”. But if it were just “cmk@mit.edu” the change would not affect me.
  
  
• In larger organizations, it might be desirable to have multiple servers share the load of handling incoming mail.

To address all of these requirements, the DNS system includes a feature that was specifically designed to support electronic mail addressing. A special mail exchange (MX) record can be set up that specifies which SMTP server should be used for mail coming in to a particular domain name. If properly configured, this allows considerable flexibility to handle the cases I described above and more. For more details, please see the description of the MX record and DNS electronic mail support.

For example, I am the owner of the “pcguide.com” domain name. E-mail can be sent to me at “pcguide.com”, but it is not actually stored on any server by that name. It is redirected to the real server where my inbox is located. This allows me to handle all incoming mail to “pcguide.com” regardless of where my mailbox is actually located.

DNS is also significant in that its “MX” resource records eliminated the need to “relay” e-mail from one SMTP server to the next to deliver it. In modern TCP/IP it is possible to send e-mail directly from the sender's SMTP server to the recipient's, making communication faster and more efficient. See the section devoted to SMTP for details.

TCP/IP Electronic Mail Addresses and Addressing 1 2 3 TCP/IP Historical and Special Electronic Mail Addressing