Apex domains cannot have CNAME records.
Question and response
Question
dig mail.intercom.io. NS
Answer
mail.intercom.io. 5 NS ns-995.awsdns-60.net.
mail.intercom.io. 5 NS ns-289.awsdns-36.com.
mail.intercom.io. 5 NS ns-1582.awsdns-05.co.uk.
mail.intercom.io. 5 NS ns-1249.awsdns-28.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0