by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568 Pub Date: 11/01/98
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Illustrated TCP/IP
by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568 Pub Date: 11/01/98 |
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The following slide shows the interaction with DNS for the mail service.
A record known as the MX record in DNS identifies a mail exchanger for the purpose of identifying hosts for recipients. A mail exchanger is a host that will either process or forward mail for the domain name. Processing means that the host will deliver it the host to which it is addressed or hand it off to another transport, such as UUCP or BITNET. Forwarding means that the host will forward the message on to the final destination or to another mail exchanger closer to the destination.
There can be multiple entries for a mail exchanger in a DNS. Each MX entry will have a precedence number beside it and this signals the sender which mail host it should try first. If the precedence value is equal among MX records, then the sender will randomly pick one from the list. Once the mail sender has successfully delivered the mail to one of the MX hosts, its job is done. It is the job of the MX host to make sure it is forwarded on to its final destination. If there are no MX records for a domain name, it is up to the mailer app as to what happens next. Some will try to deliver it to the IP address of the mail destination.
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