mail02.syd.oss-core.com has no CNAME records.
Question and response
Question
dig mail02.syd.oss-core.com. NS
Answer
The response has no answer records.
Authority
oss-core.com. 900 SOA ns1.m2core.com.au. root.m2core.com.au. 2026022403 21600 10800 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0