mta01.nrb.be has no CNAME records.
Question and response
Question
dig mta01.nrb.be. NS
Answer
The response has no answer records.
Authority
nrb.be. 51 SOA ns6.nrb.be. hostmaster.nrb.be. 2016053998 900 180 259200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0