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Unformatted text preview: .29. This is rejected in line 2. The SMTP client on vangogh then tries the next
IP address for sun: 184.108.40.206 (line 3), and it also causes an RST to be returned (line
4). file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (12 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol The SMTP client doesn't try to differentiate between the different error returns from its
active open on line 1, which is why it tries the other IP address on line 3. If the error had
been something like "host unreachable" for the first attempt, it's possible that the second
attempt could work.
If the reason the SMTP client's active open fails is because the server host is down, we
would see the client retransmit the SYN to IP address 220.127.116.11 for a total of 75
seconds (similar to Figure 18.6), followed by the client sending another three SYNs to IP
address 18.104.22.168 for another 75 seconds. After 150 seconds the client would move
on to the next MX record with the higher preference.
VRFY and EXPN Commands
The VRFY command verifies that a recipient address is OK, without actually sending
mail. EXPN is intended to expand a mailing list, without sending mail to the list. Many
SMTP implementations (such as Sendmail) consider the two the same, but we mentioned
that newer versions of Sendmail do differentiate between the two.
As a simple test we can connect to a newer version of Sendmail and see the difference.
(We have removed the extraneous Telnet client output.)
sun % telnet vangogh.cs.berkeley.edu 25
220-vangogh.CS. Berkeley. EDU Sendmail 8.1C/6.32 ready at
Tue, 3 Aug 1993 14: 59:12 -0700
220 ESMTP spoken here
250 vangogh.CS.Berkeley.EDU Hello sun.tuc.noao.edu
[22.214.171.124], pleased to meet you
550 nosuchname... User unknown
250 Richard Stevens <rstevens@vangogh.CS.Berkeley.EDU>
250 Richard Stevens <email@example.com>
First notice that we purposely ty...
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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