Apex domains cannot have CNAME records.
Question and response
Question
dig mx.post.pl. NS
Answer
mx.post.pl. 5 NS ns2.home-whs.pl.
mx.post.pl. 5 NS ns3.home-whs.pl.
mx.post.pl. 5 NS ns1.home-whs.pl.
Authority
The response has no authority records.
Additional
ns1.home-whs.pl. 5 A 62.129.250.7
ns2.home-whs.pl. 5 A 62.129.250.8
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0