mx4.comclark.com has no NS records.
NS records for this zone are defined at comclark.com.
NS records for this zone are defined at comclark.com.
Question and response
Question
dig mx4.comclark.com. NS
Answer
The response has no answer records.
Authority
comclark.com. 2560 SOA ns1.comclark.com. hostmaster.comclark.com. 2013061202 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0