SBS 2008 multiple email configurations with smart host

Hello everyone...I have what I hope is an easy questions for an SBS2008 guru.

I have an SBS server that currently is set up with a new domain name.  Lets call it is managed by the SBS server to allow them access to  The exchange mailboxes are setup to be
Heres the rub... I dont want to change the autodns management, which is why I created  However, the company already has their primary domain, and a host that hosts the POP email and accounts.
I want to setup exchange to accept  Configure a POP connector to get the mail from the ISP, and a smart host to send the mail from to the ISP.  
I essentially want to work for and to be the primary send as address.  I will also have to manually setup the email addresses for since they were setup by the ISP and not to a specific standard.

I hope this isnt too much info and someone can help me.  Thanks
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