Apex domains cannot have CNAME records.
Question and response
Question
dig espo.cz. NS
Answer
espo.cz. 5 NS mail.espo.cz.
espo.cz. 5 NS inet.faster.cz.
Authority
The response has no authority records.
Additional
inet.faster.cz. 5 A 81.19.0.66
mail.espo.cz. 5 A 81.19.8.3
inet.faster.cz. 5 AAAA 2a02:e98:0:3:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0