mail.buysub.com has no NS records.
NS records for this zone are defined at buysub.com.
NS records for this zone are defined at buysub.com.
Question and response
Question
dig mail.buysub.com. NS
Answer
The response has no answer records.
Authority
buysub.com. 86400 SOA pdns67.ultradns.com. hostmaster.cds-global.com. 2012063777 10800 3600 2419200 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0