mx-a.stcable.net has no CNAME records.
Question and response
Question
dig mx-a.stcable.net. NS
Answer
The response has no answer records.
Authority
stcable.net. 5127 SOA bind-1.sattrakt.net. webmaster.stcable.net. 2009042831 10800 3600 604800 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0