Exchange Server Email Issue

I had someone set up Exchange server on my Windows 2000 network. We only have dial up access for mail because we are out in the sticks! Any way, we have an ISP hosting our email services. We have a number of remote users that have our domain name email addresses, ie
someone@ourcompany.com. We cannot email them from the main office because the message is undeliverable and says that the email account does not exist at the organization this message was sent to. In other words the exchange server is looking for that someone@ourcompany.com email address here at our local server and can’t find it so says it doesn’t exist. Is there anyone that knows how I can fix this problem. I hope I have explained it okay.

Thanks