Exchange 2007 – cannot retrieve mail from and external POP3 server

We recently installed a new internal Exchange 2007 server and since it went live, are no longer able to retrieve mail from an external third-party POP3 server (different domain) through Outlook 2003. Our SMTP gateway receives all email first and routes it to the Exchange 2007 server for mailbox delivery.
I’ve tried setting the external POP3 domain up in the ‘Accepted Domains’ in the Hub Transport settings for the Organization, and then email can be retrieved but no longer able to be sent to that domain. The email is obviously getting to the Exchange server but it will not route it to the mailbox.
How do you you set this up to work properly in Exchange 2007?