mail.smtp24.com has no CNAME records.
Question and response
Question
dig mail.smtp24.com. NS
Answer
The response has no answer records.
Authority
smtp24.com. 1416 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2395554555 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0