Configuring SMTP behind a firewall

I have an exchange 2000 server for an internal domain (.local) behind an ISA 2000 firewall. We’ve been using a hosted e-mail server for Internet access, and the Internet e-mail service on Outlook. Yesterday, we stopped being able to send e-mail to the Internet. Our Exchange server is queing the outgoing mail, but is unable to pass it on. I’ve tried to configure the virtual SMTP server to send the mail on, and created a packet filter in ISA to accommodate this, but without success. I would appreciate any suggestions.