Exchange and ISP pop3

I have a client that has exchange 2003 that came along with 2003 SBS, they are using exchange for internal email and their ISP pop3 for external email. They have problems with sending emails through outlook 2003, they have to select which account to use depending on weather the email is internal or external. The big problem is when they want to send to both at the same time.

What kind of options do we have? One that comes to mind is to use their exchange server only, by reconfiguring it to recieve the email their ISP is currently handling.

Any advise would be great.