ns4.nrb.be has no MX records.
Question and response
Question
dig ns4.nrb.be. MX
Answer
The response has no answer records.
Authority
nrb.be. 600 SOA ns3.nrb.be. hostmaster.nrb.be. 2016053998 900 180 259200 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0