aku18.mta.exacttarget.com has no CNAME records.
Question and response
Question
dig aku18.mta.exacttarget.com. NS
Answer
The response has no answer records.
Authority
mta.exacttarget.com. 637 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 264 3 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0