Apex domains cannot have CNAME records.
Question and response
Question
dig mymxserver.com. NS
Answer
mymxserver.com. 5 NS ns01.agenturserver.de.
mymxserver.com. 5 NS ns01.agenturserver.it.
mymxserver.com. 5 NS ns01.agenturserver.co.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0