Use of Proxy server on WAN

I have 4 offices in a single NT 4.0 domain and want to use WinProxy to allow internet access and email routing. It’s set up on a PC at the main office. When I try to use it’s IP address from one of the branch offices, the user is unable to connect. Main office router is 172.16.50.1, branch offices are 172.16.70.1, 172.16.80.1, 172.16.90.1, Proxy server IP is 172.16.50.100.