Apex domains cannot have CNAME records.
Question and response
Question
dig mail.atel-mt.cz. NS
Answer
mail.atel-mt.cz. 5 NS alpha2.rdrhost.com.
mail.atel-mt.cz. 5 NS alpha1.rdrhost.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0