smtp1.gtnexus.com has no NS records.
NS records for this zone are defined at gtnexus.com.
NS records for this zone are defined at gtnexus.com.
Question and response
Question
dig smtp1.gtnexus.com. NS
Answer
The response has no answer records.
Authority
gtnexus.com. 86400 SOA edns2.ultradns.com. premiumdns.support.neustar. 2019053539 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0