Apex domains cannot have CNAME records.
Question and response
Question
dig dialog-mail.com. NS
Answer
dialog-mail.com. 5 NS ns.udag.org.
dialog-mail.com. 5 NS ns.udag.de.
dialog-mail.com. 5 NS ns.udag.net.
Authority
The response has no authority records.
Additional
ns.udag.de. 5 A 217.160.80.253
ns.udag.net. 5 A 185.132.33.253
ns.udag.org. 5 A 185.132.34.253
ns.udag.de. 5 AAAA 2001:8d8:fe:53:217:160:80:253
ns.udag.net. 5 AAAA 2607:f1c0:fe:53:185:132:33:253
ns.udag.org. 5 AAAA 2607:f1c0:fe:53:185:132:34:253
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0