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Unformatted text preview: e, headers, and body,
with changes required only in the user agents.
When a user agent passes a new mail message to its MTA, delivery is normally attempted
immediately. If the delivery fails, the MTA must queue the message and try again later.
The Host Requirements RFC recommends an initial timeout of at least 30 minutes. The
sender should not give up for at least 4-5 days. Furthermore, since delivery failures are
often transient (the recipient has crashed or there is a temporary loss of network
connectivity), it makes sense to try two connection attempts during the first hour that the
message is in the queue. 28.3 SMTP Examples
We showed normal mail delivery in the previous section, so here we'll show how MX
records are used for mail delivery, and illustrate the VRFY and EXPN commands.
MX Records: Hosts Not Directly Connected to the Internet
In Section 14.6 we mentioned that one type of resource record in the DNS is the mail file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (8 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol exchange record, called MX records. In the following example we'll show how MX
records are used to send mail to hosts that are not directly connected to the Internet. RFC
974 [Partridge 1986] describes the handling of MX records by MTAs.
The host mlfarm.com is not directly connected to the Internet, but has an MX record
that points to a mail forwarder that is on the Internet:
sun % host -a-v-t mx mlfarm.com
The following answer is not authoritative:
mlfarm.com 86388 86388
hsi86.hsi.com 172762 IN MX IN MX 10
15 hsi86.hsi.com IN
184.108.40.206 There are two MX records, each with a different preference. We expect the MTA to start
with the lower of the two preference values.
The following script shows mail being sent to this host:
sun % mail -v [email protected] arm.com -v flag to see what the MTA does
To: [email protected] arm.com
Subject: MX test message
the body of the message is typed here (not
shown) period on a lin...
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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