Exchange 2003 mail problem

I’m new to this, so bear with me. I’ve just set up SBS 2003 w/ Exchange 2003 for engineering dept. Our small business has been using an ISP for e-mail. I set up acct’s. for engineering and mailboxes. We can now e-mail each other and receive e-mail from others in the company that are not on the Exchange server, but we can’t SEND to others in the company not on Exchange. We CAN e-mail to outside the company. What have I done wrong? Can anybody help?