_spfv6.comcast.net has no MX records.
Question and response
Question
dig _spfv6.comcast.net. MX
Answer
The response has no answer records.
Authority
comcast.net. 900 SOA dns101.comcast.net. domregtech.comcastonline.com. 2008258639 7200 3600 1209600 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0