Why does a name server send no MX records?

[Jim Reid]

    >> There are some situations when a name server does not send MX
    >> records for an MX query and sends SOA records.  Is it supposed
    >> to be a rare case, or is it okay if happens often?

It depends. Getting a SOA record in the answer is a perfectly valid
response from the name server. The SOA record will usually be returned
in the Authority Section of a reply to indicate that the name and
record type being looked up definitely does not exist. [This is one of
the reasons why people should use dig for DNS troubleshooting because
you get to see what's in all the sections of the answer.] So if a mail
system is looking up a name that doesn't exist or has no MX records,
it probably means that either the mail system is broken or DNS is bust
for the domain that's being looked up or both. ie The mail system is
broken because it's asking for names that aren't in the DNS or the DNS
is broken because the names that mail systems are looking up are not
there. I'll leave it as an exercise to the reader to figure out how
common it is to find broken DNS and mail configurations. Or whether
that's OK or not. :-)