ns4.exacttarget.com has no MX records.
Question and response
Question
dig ns4.exacttarget.com. MX
Answer
The response has no answer records.
Authority
exacttarget.com. 300 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1682536669 7200 3600 1209600 21600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0