Whats happening is the mail server on Missy notices that promotem.com.au exists on Missy. It then checks to see if there is an email address called gary and when it doesn't find it, it rejects the email because it doesn't find a local sender by that name....trying to prevent spam...
So you can either set this address up on Missy, knowing that when adam replies it is going to go to the inbox on the missy server not the one on the server in AU, OR you ca set the MX for promotem.com.au to point to your mail server in AU