mail1.optus.net.au has no CNAME records.
Question and response
Question
dig mail1.optus.net.au. NS
Answer
The response has no answer records.
Authority
optus.net.au. 1668 SOA o22rrdnsme01.optus.net.au. hostmaster.optus.net.au. 2025102305 7200 1800 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0