cust-spf.exacttarget.com has no MX records.
Question and response
Question
dig cust-spf.exacttarget.com. MX
Answer
The response has no answer records.
Authority
exacttarget.com. 300 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1682536669 7200 3600 1209600 21600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0