Apex domains cannot have CNAME records.
Question and response
Question
dig de-mail.net.do. NS
Answer
de-mail.net.do. 5 NS ns-mam.ui-dns.org.
de-mail.net.do. 5 NS ns-mam.ui-dns.com.
de-mail.net.do. 5 NS ns-mam.ui-dns.de.
de-mail.net.do. 5 NS ns-mam.ui-dns.biz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0