Users cannot send email

We have Exchange 5.5 runnning on SBS 4.5. A branch office uses our mail server with Outlook Express. The users there are unable to send email outside our domain. They can send email to me and others in our @regrizlaw.com domain, but not outside. In this office, we can send outside email, but the branch office cannot. The POP3 and SMTP settings are correct. I'm guessing I have an SMTP setting screwed up somewhere, but I am not familiar enough with Exchange to find it, I guess. Any help would be greatly appreciated.