Apex domains cannot have CNAME records.
Question and response
Question
dig mymailserver.org. NS
Answer
mymailserver.org. 5 NS dns176.d.register.com.
mymailserver.org. 5 NS dns103.b.register.com.
mymailserver.org. 5 NS dns029.a.register.com.
mymailserver.org. 5 NS dns215.c.register.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0