Apex domains cannot have CNAME records.
Question and response
Question
dig mailing.nrb.be. NS
Answer
mailing.nrb.be. 5 NS ns6.nrb.be.
mailing.nrb.be. 5 NS ns4.nrb.be.
mailing.nrb.be. 5 NS ns3.nrb.be.
Authority
The response has no authority records.
Additional
ns3.nrb.be. 5 A 217.117.34.74
ns4.nrb.be. 5 A 217.117.34.75
ns6.nrb.be. 5 A 13.80.155.61
ns3.nrb.be. 5 AAAA 2a02:7100:0:301:0:0:0:100
ns4.nrb.be. 5 AAAA 2a02:7100:0:301:0:0:0:101
ns6.nrb.be. 5 AAAA 2603:1020:203:3:0:0:0:25a
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0