ns.comcastbusiness.net has no MX records.
Question and response
Question
dig ns.comcastbusiness.net. MX
Answer
The response has no answer records.
Authority
comcastbusiness.net. 900 SOA ns.comcastbusiness.net. domregtech.comcastonline.com. 2008113806 7200 300 604800 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0