Apex domains cannot have CNAME records.
Question and response
Question
dig eml100dxb.ctr.sophos.com. NS
Answer
eml100dxb.ctr.sophos.com. 5 NS ns-2002.awsdns-58.co.uk.
eml100dxb.ctr.sophos.com. 5 NS ns-647.awsdns-16.net.
eml100dxb.ctr.sophos.com. 5 NS ns-159.awsdns-19.com.
eml100dxb.ctr.sophos.com. 5 NS ns-1241.awsdns-27.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0