Could not send/receive emails from outlook express though ISA server

Then you have to forward it from one of your mailservers to your ISP's mailservers and unless you have a business account and a static IP they probably won't relay email outside of your personal email addresses.

Although I do have all of the required stuff, my own email servers, business account, various domains and servers, I'll tell you how I can forward and relay, but only through my own Busienss Static IP Addresses [which go through my ISP because all they supply is the pipe].

I use Linux as a FrontEnd Server for XP, NT, and other servers and boxes.  On one XP Home box I have MailEnable installed.  The Linux box is the gateway to the Internet, via upstream ISP, and the Windows Servers and boxes feed through the Linux box, or a router, under Linux control.

So MailEnable, on a simple XP Home box acting as the mail server for local network to Internet, and Internet to local network, has the mail server as, let's say, Brahams.Musics.com

When it sends email out to the Internet, it sends it as username@Brahams.Musics.com which shows up as username@Musics.com once it passes through the Linux Server and on out to the static IP on an unblocked port.  Were I to try and use the ISP's mail server, I would get Relay Denied. I don't because my setup is legitimate [the benefit of having your own real domain and a static IP with a business account].

Unless you have a business account, you may already be in violation of the ISP's Terms of Service by running a Server through a residential network out onto the Internet.

Even if you get it working, you should really look into getting the proper setup and account, before you get yourself on a spam list.

It used to be you could do all those things without such credentials, but then people abused it, some still are, and thus, the ISP's shut down the mail relays.

You can only send email currently using your ISP username account and password.  You cannot forge a domainname either.  You have to Register a real domain, what they call a Fully Qualified Domain Name [FQDN].  From there, you have some work to do.  Offers for free domains, free DNS, and free email servers are bogus.  They are "all" recognized as the major source of spam, currently the U.S. is the biggest spammer with over 50% of all the spam email in the world coming from the U.S.

You have the luck to have the Servers.  Now invest in making a real domain and network.