sendmail, 4th Edition

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 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 ...