External POP in Exchange 2007

I need help with configuring my Exchange 2007. In my company we are using companyname.local domain and exchange is working just fine but also have registered domain companyname.com. I need this emails that arrive at my ISP who is hosting my companyname.com domain go receive in my local exchange account. I need somehow to take them from my ISP via POP or IMAP since I have a dynamic local IP address

Thanks In Advance