Apex domains cannot have CNAME records.
Question and response
Question
dig mail-forwarder.io. NS
Answer
mail-forwarder.io. 5 NS ns-213.awsdns-26.com.
mail-forwarder.io. 5 NS ns-590.awsdns-09.net.
mail-forwarder.io. 5 NS ns-1476.awsdns-56.org.
mail-forwarder.io. 5 NS ns-1724.awsdns-23.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0