mx1.bigair.com.au has no CNAME records.
Question and response
Question
dig mx1.bigair.com.au. NS
Answer
The response has no answer records.
Authority
bigair.com.au. 10800 SOA ns1.bigair.com.au. serveradmin.bigair.net.au. 2026022601 14400 7200 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0