why mail server doesn’t work after installing ISA

I have mail server IMAIL installed on my Windows 2000 server. Now hope to install ISA server 2000 on it.
After installation of ISA, I create two policies, one for HTTP and another for Mail, allow Intranet clients to visit Internet, and use mail service.
But I find the Mail Server itself doesn’t work. How can I ensure the SMTP and POP3 service of the ISA server work properly?