mail.mxthunder.com has no CNAME records.
Question and response
Question
dig mail.mxthunder.com. NS
Answer
The response has no answer records.
Authority
mxthunder.com. 5006 SOA ns.dnsbox.net. hostmaster.dnsbox.net. 2026021706 10000 3600 604800 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0