mail.optus.com.au has no CNAME records.
Question and response
Question
dig mail.optus.com.au. NS
Answer
The response has no answer records.
Authority
optus.com.au. 358 SOA dns0.optus.net.au. hostmaster.optus.net.au. 2026030213 28800 7200 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0