postmaster usually exists and is aliased to root the fact that it bounce with that message though means that it didn't even get as far as looking for the user.
Your MX record is correct the way it is. MX records should resolve to A records which in turn resolve to IPs. They should not directly resolve to IPs.
You just need to add "bjc.id.au" to local-host-names on a new line:
# local-host-names - include all aliases for your machine here.
I can't remember but I think you need to restart sendmail. If you want me to send a test message just let me know.