Apex domains cannot have CNAME records.
Question and response
Question
dig antispamgw.host. NS
Answer
antispamgw.host. 5 NS ns4.aname.net.
antispamgw.host. 5 NS ns3.aname.se.
antispamgw.host. 5 NS ns.aname.net.
antispamgw.host. 5 NS ns2.aname.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0