inbound.s1.exacttarget.com has no CNAME records.
Question and response
Question
dig inbound.s1.exacttarget.com. NS
Answer
The response has no answer records.
Authority
s1.exacttarget.com. 300 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1680012216 7200 3600 1209600 21600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0