s50.exacttarget.com has no MX records.
Question and response
Question
dig s50.exacttarget.com. MX
Answer
The response has no answer records.
Authority
s50.exacttarget.com. 300 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1680024972 7200 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0