Apex domains cannot have CNAME records.
Question and response
Question
dig mx-relay.com. NS
Answer
mx-relay.com. 5 NS ns3.witxl.nl.
mx-relay.com. 5 NS ns1.witxl.nl.
mx-relay.com. 5 NS ns2.witxl.nl.
Authority
The response has no authority records.
Additional
ns1.witxl.nl. 5 A 164.138.28.111
ns2.witxl.nl. 5 A 164.138.31.75
ns3.witxl.nl. 5 A 80.211.177.164
ns1.witxl.nl. 5 AAAA 2a02:2770:3:0:21a:4aff:fe35:9495
ns2.witxl.nl. 5 AAAA 2a02:2770:5:0:21a:4aff:fe96:a01a
ns3.witxl.nl. 5 AAAA 2a00:6d40:72:60a4:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0