mx4.cogentco.com has no CNAME records.
Question and response
Question
dig mx4.cogentco.com. NS
Answer
The response has no answer records.
Authority
cogentco.com. 5472 SOA auth1.dns.cogentco.com. dns.cogentco.com. 2026020802 10800 3600 604800 43200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0