mailnt01.ntcloudbox.com has no CNAME records.
Question and response
Question
dig mailnt01.ntcloudbox.com. NS
Answer
The response has no answer records.
Authority
ntcloudbox.com. 772 SOA ns1.totonline.net. please_set_email.absolutely.nowhere. 285 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0