mail.arcom.net.pl has no CNAME records.
Question and response
Question
dig mail.arcom.net.pl. NS
Answer
The response has no answer records.
Authority
arcom.net.pl. 3207 SOA dns.home.pl. admin.home.pl. 1574234629 14400 7200 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0