mx1h1.comcast.net has no CNAME records.
Question and response
Question
dig mx1h1.comcast.net. NS
Answer
The response has no answer records.
Authority
comcast.net. 486 SOA dns101.comcast.net. domregtech.comcastonline.com. 2008258639 7200 3600 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0