ns3.dnsbackup.net has no MX records.
Question and response
Question
dig ns3.dnsbackup.net. MX
Answer
The response has no answer records.
Authority
dnsbackup.net. 86400 SOA ns1.dnsbackup.net. root.dnsbackup.net. 2026021701 86400 7200 1857600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0