mail.smtp40.com has no CNAME records.
Question and response
Question
dig mail.smtp40.com. NS
Answer
The response has no answer records.
Authority
smtp40.com. 1475 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2397121617 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0