Wildcard MX records almost never have any appropriate use on the Internet. They
are often misunderstood and are often used just to save the effort of typing hundreds
of MX records. They do, however, have legitimate uses behind firewall machines and
on networks not connected to the Internet.
9.3.6 What? They Ignore MX Records?
Many older MTAs on the network ignore MX records. Some pre-Solaris Sun sites, for
example, wrongly run the non-MX version of sendmail when they should use /usr/lib/
sendmail.mx. Some Solaris sites wrongly do all host lookups with NIS when they
should list dns on the hosts line of their /etc/nsswitch.conf file. Because of these and
other mistakes, you will occasionally find some sites that insist on sending mail to a
host even though that host has been explicitly MX??™d to another.
To illustrate why this is bad, consider a UUCP host that has only an MX record. It
has no A record because it is not on the network:
uuhost IN MX 10 uucpserver
Here, mail to uuhost will be sent to uucpserver, which will forward the message to
uuhost withUUCP software. An attempt to ignore this MX record will fail because
uuhost has no other records. Similar problems can arise for printers with direct network
connections, terminal servers, and even workstations that don??™t run an SMTP
daemon such as sendmail.
If you believe in DNS and disdain sites that don??™t, you can simply ignore the offending
sites.
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