Apex domains cannot have CNAME records.
Question and response
Question
dig webmail.app.eventex.xyz. NS
Answer
webmail.app.eventex.xyz. 5 NS ns6.afternic.com.
webmail.app.eventex.xyz. 5 NS ns5.afternic.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0