smtp3.emailopen.com has no CNAME records.
Question and response
Question
dig smtp3.emailopen.com. NS
Answer
The response has no answer records.
Authority
emailopen.com. 1800 SOA frank.ns.cloudflare.com. dns.cloudflare.com. 2397284433 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0