mx01.mailcluster.com.au has no CNAME records.
Question and response
Question
dig mx01.mailcluster.com.au. NS
Answer
The response has no answer records.
Authority
mailcluster.com.au. 2463 SOA ns1.digitalpacific.com.au. hostmaster.mailcluster.com.au. 2025102939 7200 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0