mail.busum.net has no CNAME records.
Question and response
Question
dig mail.busum.net. NS
Answer
The response has no answer records.
Authority
busum.net. 120 SOA ns2.busum.net. root.ns3008652.ip-149-202-66.eu. 2026030403 3600 300 3600 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0