Configuring Exchange Front-End Servers
A front-end server is an ordinary Exchange server until it is configured as a front-end server. A front-end server must not host any users or public folders.
A front-end server must be a member of the same Exchange organization as the back-end servers (therefore, a member of the same Windows® forest).
For detailed instructions about how to designate an Exchange server as a front-end server, see How to Designate a Front-End Server.