smtp1.gtnexus.com has no MX records.
Question and response
Question
dig smtp1.gtnexus.com. MX
Answer
The response has no answer records.
Authority
gtnexus.com. 86400 SOA edns2.ultradns.com. premiumdns.support.neustar. 2019053536 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0