As a heads up to every one, when you upgrade from Exchange 5 to 2000 or from 2000 to service pack three it turns Exchange into an open mail relay, you need to go into your server properties and close this. MS will fix this issue in the first post SP3 hotfix.

My suggestion would be to use a *nix based mail gateway and sit your exchange server behind this blocking the problem all together. As long as you make sure the *nix mail client you use is configured properly.