s4.exacttarget.com has no MX records.
Question and response
Question
dig s4.exacttarget.com. MX
Answer
The response has no answer records.
Authority
s4.exacttarget.com. 60 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1679946536 7200 3600 1209600 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0