Gateways are interfaces that connect electronic messaging systems with different message and address formats. In their simplest form, they receive mail messages, translate them from one messaging system's format to another, map the address between the two systems, and then deliver the translated messages. A gateway can translate messages between mail systems within a single organization, or between an organization and an outside mail system.
For detailed information about gateways, see the Microsoft Exchange Server documentation in the "Database and Messaging Services" section of the Microsoft Platform SDK.