Apex domains cannot have CNAME records.
Question and response
Question
dig mail.cloudfast.eu. NS
Answer
mail.cloudfast.eu. 5 NS sl1.sedo.com.
mail.cloudfast.eu. 5 NS sl2.sedo.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0