Backboning is the process of transmitting messages between two similar messaging systems using an intermediate system of a different type. It is an important feature that should be offered by all Microsoft Exchange Server gateways. The goal of backboning is to send messages between Microsoft Exchange Server sites in different organizations as easily as you do between two users on the same site. A typical example is that of Microsoft Exchange Server sites at widely separated locations throughout the country. Mail users can send messages to others in distant cities by using the Internet as a backbone.
This section discusses using a foreign system as a backbone between two Microsoft Exchange Server systems. You can also use a Microsoft Exchange Server backbone between two foreign systems.