Multiple Domains in exchange 2003

I have recently changed my domain from companyname.on.net to companyname.net.au
I wish my to configure the server to accept the emails from both domains. the companyname is the server domain without .local as i have macs on board therefore the company name will have emails from .on.net plus .net.au can anyone help me to configure this or is exchange not able to do this. the companyname is a constant and the only variable is the .net.au and the on.net.

John