s51.exacttarget.com has no MX records.
Question and response
Question
dig s51.exacttarget.com. MX
Answer
The response has no answer records.
Authority
s51.exacttarget.com. 300 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1679951115 7200 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0