mail6.smtp12.com has no CNAME records.
Question and response
Question
dig mail6.smtp12.com. NS
Answer
The response has no answer records.
Authority
smtp12.com. 947 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2395382460 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0