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
Solaris sites wrongly do all host lookups with NIS
when they should list
dns on the
hosts line of their
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 with UUCP 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. 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, Add STARTTLS Support to Your mc File, notes that obeying MX records is mandatory. RFC1123 ...