Apex domains cannot have CNAME records.
Question and response
Question
dig mail.tue.nl. NS
Answer
mail.tue.nl. 5 NS ns02.tue.nl.
mail.tue.nl. 5 NS ns01.tue.nl.
Authority
The response has no authority records.
Additional
ns01.tue.nl. 4 A 131.155.3.20
ns02.tue.nl. 4 A 131.155.3.30
ns01.tue.nl. 4 AAAA 2001:610:b41:1000:0:0:0:20
ns02.tue.nl. 4 AAAA 2001:610:b41:1001:0:0:0:30
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0