s7.exacttarget.com has no MX records.
Question and response
Question
dig s7.exacttarget.com. MX
Answer
The response has no answer records.
Authority
s7.exacttarget.com. 300 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1680014287 7200 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0