mailbox.xtra.pl has no CNAME records.
Question and response
Question
dig mailbox.xtra.pl. NS
Answer
The response has no answer records.
Authority
xtra.pl. 3682 SOA ns1.xtra.pl. root.ns1.xtra.pl. 2025080101 10800 3600 1814400 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0