In this case, the mail will fail if your MX??™d host doesn??™t run a sendmail daemon
(or another MTA). This is not as nasty as it sounds. There is actually considerable support
for this approach; failure to obey MX records is a clear violation of published network
protocols. RFC1123, Host Requirements, section 5.3.5, notes that obeying MX
records is mandatory. RFC1123 has existed for more than 12 years.
On the one hand, to ensure that all mail is received, even on a workstation whose
mail is MX??™d elsewhere, you can run the sendmail daemon on every machine. On the
other hand, to ensure that all mail is received, even for hosts that are not machines
(like uuhost earlier) you can assign each such host an IP address that is the IP address
of your mail server.
9.3.7 Caching MX Records
Although you are not required to have MX records for all hosts, there is a good reason
to consider doing so. To illustrate, consider the following host that has only an A
record:
hostB IN A 123.45.67.8
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Copyright ?© 2007 O??™Reilly & Associates, Inc. All rights reserved.
9.3 Set Up MX Records | 337
When V8.12 and above sendmail first look up this host, they ask the name server for
that host??™s MX records. Because there are none, that request comes back empty. The
sendmail program must then make a second lookup for the IP address.
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