mail.buysub.com has no CNAME records.
Question and response
Question
dig mail.buysub.com. NS
Answer
The response has no answer records.
Authority
buysub.com. 7006 SOA pdns67.ultradns.com. hostmaster.cds-global.com. 2012063777 10800 3600 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0