Apex domains cannot have CNAME records.
Question and response
Question
dig smtp25.com. NS
Answer
smtp25.com. 5 NS ns-1353.awsdns-41.org.
smtp25.com. 5 NS ns-533.awsdns-02.net.
smtp25.com. 5 NS ns-254.awsdns-31.com.
smtp25.com. 5 NS ns-1553.awsdns-02.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0