Reverse Hosting

Reverse Hosting Can anyone tell me what this is please (does it go under a different name?), and maybe how I do it? I have a proxy that I now want to put in series with our router. We have 512 public addresses. We want to make the internal network private but our email server (Imail) uses a public address and will be behind the proxy. The helpful chap at RM said "you want reverse hosting on your squid box" - cant find anything about it. Kind regards Adrian Wells