Apex domains cannot have CNAME records.
Question and response
Question
dig titan.email. NS
Answer
titan.email. 5 NS ns-1806.awsdns-33.co.uk.
titan.email. 5 NS ns-451.awsdns-56.com.
titan.email. 5 NS ns-953.awsdns-55.net.
titan.email. 5 NS ns-1463.awsdns-54.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0