Apex domains cannot have CNAME records.
Question and response
Question
dig cleanmail.eu. NS
Answer
cleanmail.eu. 5 NS ns3.rrpproxy.net.
cleanmail.eu. 5 NS ns1.rrpproxy.net.
cleanmail.eu. 5 NS ns2.rrpproxy.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0