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Unformatted text preview: ped the wrong hostname on the HELO command: bsdi
instead of sun. Most SMTP servers take the IP address of the client and perform a DNS
pointer query (Section 14.5) and compare the hostnames. This allows the server to log the
client connection based on the IP address, not the name that a user might have mistyped. file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (13 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol Some servers respond with humorous messages, such as "You are a charlatan," or "why
do you call yourself...". We see in this example that this server just prints our real domain
name from the pointer query along with our IP address.
We then type a VRFY command for an invalid name, and the server responds with a 550
error. Next we type a valid name, and the server responds with the usemame on the local
host. Next we try the EXPN command and get a different response. The EXPN command
determines that the mail for this user is being forwarded, and prints the forwarding
Many sites disable the VRFY and EXPN commands, sometimes for privacy, and
sometimes in the belief that it's a security hole. For example, we can try these commands
with the SMTP server at the White House:
sun % telnet whitehouse.gov 25
220 whitehouse.gov SMTP/smap Ready.
250 (sun.tuc.noao.edu) pleased to meet you.
500 Command unrecognized
500 Command unrecognized 28.4 SMTP Futures
Changes are taking place with Internet mail. Recall the three pieces that comprise Internet
mail: the envelope, headers, and body. New SMTP commands are being added that affect
the envelope, non-ASCII characters can be used in the headers, and structure is being
added to the body (MIME). In this section we consider the extensions to each of these
three pieces in order.
Envelope Changes: Extended SMTP
RFC 1425 [Klensin et al. 1993a] defines the framework for adding extensi...
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