s6.exacttarget.com has no MX records.
Question and response
Question
dig s6.exacttarget.com. MX
Answer
The response has no answer records.
Authority
s6.exacttarget.com. 60 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1680013781 7200 3600 1209600 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0