Apex domains cannot have CNAME records.
Question and response
Question
dig mta.tessian-gw.com. NS
Answer
mta.tessian-gw.com. 1 NS ns-484.awsdns-60.com.
mta.tessian-gw.com. 1 NS ns-1163.awsdns-17.org.
mta.tessian-gw.com. 1 NS ns-564.awsdns-06.net.
mta.tessian-gw.com. 1 NS ns-1656.awsdns-15.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0