mail.sisgroup.com.au has no MX records.
Question and response
Question
dig mail.sisgroup.com.au. MX
Answer
The response has no answer records.
Authority
sisgroup.com.au. 300 SOA ns1.sisgroup.com.au. admin.sisgroup.com.au. 1597383665 21600 3600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0