question on Mail server + ISA server

I have one mail server with IMAIL installed. Now hope to install ISA server 2000 on the same server.
I cannot install schema into AD, due to an error message: “the format of the specified computer name is invalid”; so it is a stand alone server – using 2 NICs, one for Extranet, the other for Intranet.
After installation of ISA, I create two policies, one for HTTP and another for SMTP & POP3, allowing Intranet clients to visit Internet, and use mail services.
But after then I find the Mail Server itself doesn’t work properly, whilst all clients can visit Internet.
The mail server cannot send out or receive mails for the clients.
How can I ensure the SMTP and POP3 service of the mail server work properly after installing ISA server2000?