ns.coredns.org has no MX records.
Question and response
Question
dig ns.coredns.org. MX
Answer
The response has no answer records.
Authority
coredns.org. 3600 SOA ns1.server.eu. postmaster.server.eu. 2012071910 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0