mail20.m.freebit.net has no CNAME records.
Question and response
Question
dig mail20.m.freebit.net. NS
Answer
The response has no answer records.
Authority
m.FreeBit.NET. 600 SOA NS21.FreeBit.NET. root.FreeBit.NET. 2018030400 3600 900 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0