mail.backup.infoplus.com.pl has no CNAME records.
Question and response
Question
dig mail.backup.infoplus.com.pl. NS
Answer
The response has no answer records.
Authority
backup.infoplus.com.pl. 10318 SOA dns1.panelplus.pl. bok.infoplus.com.pl. 2023022004 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0