mx4.cogentco.com has no NS records.
NS records for this zone are defined at cogentco.com.
NS records for this zone are defined at cogentco.com.
Question and response
Question
dig mx4.cogentco.com. NS
Answer
The response has no answer records.
Authority
cogentco.com. 43200 SOA auth1.dns.cogentco.com. dns.cogentco.com. 2026020802 10800 3600 604800 43200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0