mail15.smtp12.com has no CNAME records.
Question and response
Question
dig mail15.smtp12.com. NS
Answer
The response has no answer records.
Authority
smtp12.com. 822 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2395382460 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0