Apex domains cannot have CNAME records.
Question and response
Question
dig rs.smtp.com. NS
Answer
rs.smtp.com. 5 NS ns-574.awsdns-07.net.
rs.smtp.com. 5 NS ns-1663.awsdns-15.co.uk.
rs.smtp.com. 5 NS ns-209.awsdns-26.com.
rs.smtp.com. 5 NS ns-1435.awsdns-51.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0