Apex domains cannot have CNAME records.
Question and response
Question
dig spamfilterserver.com. NS
Answer
spamfilterserver.com. 5 NS ns3.thednscompany.com.
spamfilterserver.com. 5 NS ns1.thednscompany.com.
spamfilterserver.com. 5 NS ns2.thednscompany.com.
Authority
The response has no authority records.
Additional
ns1.thednscompany.com. 5 A 185.145.63.233
ns2.thednscompany.com. 5 A 37.48.115.145
ns3.thednscompany.com. 5 A 149.210.192.124
ns1.thednscompany.com. 5 AAAA 2a02:cc9:0:63:0:0:0:233
ns2.thednscompany.com. 5 AAAA 2001:1af8:4700:a112:13:0:0:50
ns3.thednscompany.com. 5 AAAA 2a01:7c8:aab5:5b3:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0