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Unformatted text preview: mand to see how this name is defined to the DNS: file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (6 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol sun % host mailhost
220.127.116.11 canonical name
its real IP address If the host used as the relay changes in the future, only its DNS name need change-the
mail configuration of all the individual systems does not change.
Most organizations are using relay systems today. Figure 28.3 is a revised picture of
Internet mail (Figure 28.2), taking into account that both the sending host and the final
receiving host probably use a relay host. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (7 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol Figure 28.3 Internet electronic mail, with a relay system at both ends.
In this scenario there are four MTAs between the sender and receiver. The local MTA on
the sender's host just delivers the mail to its relay MTA. (This relay MTA could have a
hostname of mailhost in the organization's domain.) This communication uses SMTP
across the organization's local internet. The relay MTA in the sender's organization then
sends the mail to the receiving organization's relay MTA across the Internet. This other
relay MTA then delivers the mail to the receiver's host, by communication with the local
MTA on the receiver's host. All the MTAs in this example use SMTP, although the
possibility exists for other protocols to be used.
One feature of SMTP is that it uses NVT ASCII for everything: the envelope, the headers,
and the body. As we said in Section 26.4, this is a 7-bit character code, transmitted as 8bit bytes, with the high-order bit set to 0.
In Section 28.4 we discuss some newer features of Internet mail, extended SMTP and
multimedia mail (MIME), that allow the sending and receiving of data such as audio and
video. We'll see that MIME works with NVT ASCII for the envelop...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12