Apex domains cannot have CNAME records.
Question and response
Question
dig authsmtp.com. NS
Answer
authsmtp.com. 5 NS ns1.dmpriest.net.uk.
authsmtp.com. 5 NS ns1.auth-dns.com.
authsmtp.com. 5 NS ns2.dmpriest.net.uk.
authsmtp.com. 5 NS ns3.dmpriest.net.uk.
authsmtp.com. 5 NS ns0.dmpriest.net.uk.
authsmtp.com. 5 NS ns2.auth-dns.com.
Authority
The response has no authority records.
Additional
ns0.dmpriest.net.uk. 5 A 62.13.128.10
ns1.auth-dns.com. 5 A 192.174.68.100
ns1.dmpriest.net.uk. 5 A 62.13.128.20
ns2.auth-dns.com. 5 A 176.97.158.100
ns2.dmpriest.net.uk. 5 A 62.13.159.10
ns3.dmpriest.net.uk. 5 A 72.52.72.35
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0