mail.backupexpert.pl has no CNAME records.
Question and response
Question
dig mail.backupexpert.pl. NS
Answer
The response has no answer records.
Authority
backupexpert.pl. 60 SOA dns113.ovh.net. tech.ovh.net. 2025050512 86400 3600 3600000 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0