ns2.mktdns.com has no MX records.
Question and response
Question
dig ns2.mktdns.com. MX
Answer
The response has no answer records.
Authority
mktdns.com. 1800 SOA ns1.marketo.com. ops.localhost. 2025100200 1800 1800 604800 1800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0