In our current situation we have 2 email servers. Internally, we have a Microsoft Mail server
that handles all our mail between employees. Every employee has an account on that server.
Externally, our ISP is hosting our SMTP server to handle our Internet mail. Only certain employees
have accounts with our ISP.

Here is the situation. We want to replace Microsoft Mail with a Linux solution. Sendmail, qmail, postfix,
etc. doesn't matter. Every employee will need an account on this server to handle the internal mail system.
We also want this machine to pull email in from our ISP's POP server. Here is the problem. Since this
machine will be running SMTP, how do we restrict which employees have the ability to send mail out of our
system? We don't want everyone sending messages to the Internet, but some people still have to. But everyone
still needs to be able to send internal messages.