Apex domains cannot have CNAME records.
Question and response
Question
dig smtp.software. NS
Answer
smtp.software. 5 NS nsn2.mijndomein.nl.
smtp.software. 5 NS nsn1.mijndomein.nl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0