_spf.mdp.comcast.net has no CNAME records.
Question and response
Question
dig _spf.mdp.comcast.net. NS
Answer
The response has no answer records.
Authority
mdp.comcast.net. 900 SOA dns101.comcast.net. dnsadmin.comcast.net. 148 7200 3600 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0