Apex domains cannot have CNAME records.
Question and response
Question
dig mail.cention.com. NS
Answer
mail.cention.com. 5 NS ns-1608.awsdns-09.co.uk.
mail.cention.com. 5 NS ns-1075.awsdns-06.org.
mail.cention.com. 5 NS ns-356.awsdns-44.com.
mail.cention.com. 5 NS ns-732.awsdns-27.net.
Authority
The response has no authority records.
Additional
ns-356.awsdns-44.com. 4 A 205.251.193.100
ns-732.awsdns-27.net. 4 A 205.251.194.220
ns-1075.awsdns-06.org. 5 A 205.251.196.51
ns-1608.awsdns-09.co.uk. 4 A 205.251.198.72
ns-356.awsdns-44.com. 4 AAAA 2600:9000:5301:6400:0:0:0:1
ns-732.awsdns-27.net. 4 AAAA 2600:9000:5302:dc00:0:0:0:1
ns-1075.awsdns-06.org. 5 AAAA 2600:9000:5304:3300:0:0:0:1
ns-1608.awsdns-09.co.uk. 4 AAAA 2600:9000:5306:4800:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0