Apex domains cannot have CNAME records.
Question and response
Question
dig smartemailing-mta.com. NS
Answer
smartemailing-mta.com. 5 NS becky.ns.cloudflare.com.
smartemailing-mta.com. 5 NS ram.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0