How many mail servers are going to be on this network?
You could just set everyone up with an account ... and when they need to send mail, they can send it locally to username@localhost. They'll get their mail from the same machine as they sent it from.
Also take a look at /etc/hosts. You could map specific internal mail servers to a specific host. I'll play around with this on Monday and see if I can replicate what you want to do.