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