SBS 2003 – Email externals users using same domain name

Emailing coworkers who also using the same SBS server is no problem. The outgoing SMTP server recoginizes that their Inbox is on the server and delivers the email.

What about a coworker who is not onsite. They independently POP their email from the same POP server that the SBS POP3 connector does.

What currently is happening (by default) is that when a internal user wants to send a message to the external user they get a message something like “Undeliverable, user does not exist”.

How do I configure Exchange on SBS 2003 to send the email to the ISP for delivery?