Apex domains cannot have CNAME records.
Question and response
Question
dig apple.relay.mailchannels.net. NS
Answer
apple.relay.mailchannels.net. 5 NS ns-954.awsdns-55.net.
apple.relay.mailchannels.net. 5 NS ns-369.awsdns-46.com.
apple.relay.mailchannels.net. 5 NS ns-1660.awsdns-15.co.uk.
apple.relay.mailchannels.net. 5 NS ns-1186.awsdns-20.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0