Apex domains cannot have CNAME records.
Question and response
Question
dig mx1.arsmtp.com. NS
Answer
mx1.arsmtp.com. 5 NS mdns2.appriver.com.
mx1.arsmtp.com. 5 NS mdns1.appriver.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0