Apex domains cannot have CNAME records.
Question and response
Question
dig postifi.posticloud.fi. NS
Answer
postifi.posticloud.fi. 5 NS ns-325.awsdns-40.com.
postifi.posticloud.fi. 5 NS ns-983.awsdns-58.net.
postifi.posticloud.fi. 5 NS ns-1180.awsdns-19.org.
postifi.posticloud.fi. 5 NS ns-1935.awsdns-49.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0