is that the only solution?
Before you come up with a solution, you need to properly understand the problem. I don't see that anyone has tried to do that.
You need to start by reading carefully the bounce message.
...
<rcausey@ausnetit-gw.ausnetit.com.au>:
Recipient unknown
...
What did "ausnetit-gw" come from? Nobody seems to have pondered that question.
If we look in the body of the undelivered message, we see:
...
Delivered-To: alias-localdelivery-rcausey@tanknology.com.au
...
To: <rcausey@tanknology.com.au>
...
That means that the message was delivered via the .qmail file of the rcausey user of the tanknology.com.au server. But that wasn't a final delivery, because you received a bounce message.
I expect you will find that the mail of the rcausey user is being forwarded to rcausey@ausnetit-gw.ausnetit.com.au, and the forwarding is unsuccessful, because the usnetit-gw.ausnetit.com.au system says "Recipient unknown".