Exchange, External Mail and POP3 Connector

Hello everyone,

Fairly new to network administration and I have taken on a volunteer project at a local non-profit organization. They currently get mail through a 3rd party ISP that is provided to them by the main headquarters of the non-profit organization. Internally, they are responsible for their own IT. As such, they purchased a server with SBS 2008 and have started to use Exchange 2007.

The Setup:
1. Mail is handled at mail.non-profit.org
2. Server is setup to be non-profit.local and contains self-issued CA for non-profit.org
3. POP3 Connector downloads mail from mail.non-profit.org and places in appropriate mailbox
4. Mail is sent from SBS 2008 server as user[at]non-profit[.]org if replied to would end back at mail.non-profit.org

Are there any seasoned network administrators that might be able to offer me some advice or direction in managing this scenario?

The problem with this setup is that the public IP of said non-profit does not match the IP of mail.non-profit.org leaving some domains no other choice than to reject messages sent from SBS 2008 server.