mail.nr.net has no CNAME records.
Question and response
Question
dig mail.nr.net. NS
Answer
The response has no answer records.
Authority
nr.net. 1113 SOA ns.nr.net. hostmaster.nr.net. 2021061225 3600 600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0