using sendmail with virtual network

[jessrs0290]

I have a virtual network setup. My external IP is a 68.xx IP and the home network machines are all 192.168....'s. How can I (what commands do I use) using UML to configure sendmail so that the local machines can only accept mail from other local machines. They cant accept mail from outside the private network.
I need to also fix it so that if someone tries to send mail to a local machine, it will go to the 68. machine instead.