Apex domains cannot have CNAME records.
Question and response
Question
dig t-email.cz. NS
Answer
t-email.cz. 5 NS eisa.telekom.sk.
t-email.cz. 5 NS idunn.t-mobile.cz.
t-email.cz. 5 NS freya.t-mobile.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0