mxbackup.cuci.nl has no MX records.
Question and response
Question
dig mxbackup.cuci.nl. MX
Answer
The response has no answer records.
Authority
cuci.nl. 64 SOA ns1.cuci.nl. hostmaster.cuci.nl. 2015010364 131073 2050 2097152 64
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0