s10.exacttarget.com has no MX records.
Question and response
Question
dig s10.exacttarget.com. MX
Answer
The response has no answer records.
Authority
s10.exacttarget.com. 300 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1680015684 7200 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0