mxbackup.routing.net has no NS records.
NS records for this zone are defined at routing.net.
NS records for this zone are defined at routing.net.
Question and response
Question
dig mxbackup.routing.net. NS
Answer
The response has no answer records.
Authority
routing.net. 900 SOA ns1.routing.net. hostmaster.routing.net. 2026021801 14400 3600 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0