mx03.routing.net has no CNAME records.
Question and response
Question
dig mx03.routing.net. NS
Answer
The response has no answer records.
Authority
routing.net. 674 SOA ns1.routing.net. hostmaster.routing.net. 2026021801 14400 3600 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0