Apex domains cannot have CNAME records.
Question and response
Question
dig cleanmail.ch. NS
Answer
cleanmail.ch. 5 NS ns4.cleanmail.ch.
cleanmail.ch. 5 NS ns5.cleanmail.ch.
cleanmail.ch. 5 NS ns3.cleanmail.ch.
Authority
The response has no authority records.
Additional
ns3.cleanmail.ch. 5 A 208.94.148.13
ns4.cleanmail.ch. 5 A 208.80.124.13
ns5.cleanmail.ch. 5 A 208.80.126.13
ns3.cleanmail.ch. 5 AAAA 2600:1800:5:0:0:0:0:1
ns4.cleanmail.ch. 5 AAAA 2600:1801:6:0:0:0:0:1
ns5.cleanmail.ch. 5 AAAA 2600:1802:7:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0