Unable to Access OWA from Outside ISA FW

I am unable to access OWA from Outside ISA firewall where as I am able to accees Web site on same server having exchange 2003.
I am having Exchange 2003 on Windows 2000 server and ISA 2000 Server on seperate machine with Windows 2000 server.
OWA work fine from inside the ISA Firewall. I have Port 25 and 80 Open on ISA firewall.