Question : DNS record for 'new' accepted Exchange domain

We have seveal registerd domain names, our main one being in the format of @my-company.com.  We have offices in Australia and have the equivalent @my-company.com.au domain registered, so I'd like to start using it for mail.  I've added an MX record pointing @my-company.com.au mail to our mail server and added the accepted domian in Exchange 2007.....my question is, do I need to create a DNS record (be it internal or external) to handle reverse DNS lookup?

Answer : DNS record for 'new' accepted Exchange domain

You always want to have an external reverse DNS record as a lot of mail servers will see if you do not have a correct one and drop your inbound mail as spam.  That is maintained by the ISP for the .com.au domain.