smtp2.gtnexus.com has no CNAME records.
Question and response
Question
dig smtp2.gtnexus.com. NS
Answer
The response has no answer records.
Authority
gtnexus.com. 8169 SOA edns2.ultradns.com. premiumdns.support.neustar. 2019053537 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0