_spf.comcast.com has no CNAME records.
Question and response
Question
dig _spf.comcast.com. NS
Answer
The response has no answer records.
Authority
comcast.com. 481 SOA dns101.comcast.net. domregtech.comcastonline.com. 2009104323 7200 3600 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0