Apex domains cannot have CNAME records.
Question and response
Question
dig xsend6.email. NS
Answer
xsend6.email. 5 NS ns-1480.awsdns-57.org.
xsend6.email. 5 NS ns-207.awsdns-25.com.
xsend6.email. 5 NS ns-723.awsdns-26.net.
xsend6.email. 5 NS ns-1857.awsdns-40.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0