internalmx.smtpcorp.com has no CNAME records.
Question and response
Question
dig internalmx.smtpcorp.com. NS
Answer
The response has no answer records.
Authority
smtpcorp.com. 1800 SOA ada.ns.cloudflare.com. dns.cloudflare.com. 2397305854 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0