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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 Addressing and Address Resolution
(Page 3 of 3)
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:
The Mail Exchange (MX) DNS Record
- 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 firstname.lastname@example.org than email@example.com.
- 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 firstname.lastname@example.org. But if it were just email@example.com
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.
the section devoted to SMTP for details.
Key Concept: Some form of addressing is required for all network communication; since electronic mail is user-oriented, e-mail addresses are based on users as well. In modern TCP/IP e-mail, standard addresses consist of a user name, which specifies who the recipient is, and a domain name, which specifies the DNS domain where the user is located. A special DNS mail exchange (MX) record is set up for each domain that accepts e-mail, so a sending SMTP server can determine what SMTP server it should use to send mail to a particular recipient.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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