Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-eu28.advancedhosters.com. NS
Answer
smtp-eu28.advancedhosters.com. 1 NS ns1.advancedhosters.com.
smtp-eu28.advancedhosters.com. 1 NS ns2.advancedhosters.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0