Overview

In its simplest form, a gateway is an interface that connects one electronic messaging system to another electronic messaging system that has a different message and address format. The gateway receives the mail message, translates it from one format to the other format, maps addresses between the two systems, and then delivers the translated message.

A Microsoft Exchange Server gateway is a MAPI client application. This means that the gateway uses MAPI to communicate with Microsoft Exchange Server. The gateway can access MAPI folders, transports, and other MAPI-based services. Gateways run on computers using the Microsoft Windows NT Server operating system.

A Microsoft Exchange Server gateway

Two possible scenarios for using a gateway are:

• Between mail systems within a single organization Suppose your company has been using the PROFS system for years and now plans to convert to Microsoft Exchange Server. You can avoid business disruption by installing a gateway to connect the old and new mail systems. With the gateway, you can gradually retrain and migrate your current users to Microsoft Exchange Server, upgrade their systems, and add new users to the system. During the migration process, all employees will be able to communicate by e-mail. In such a scenario, the old mail system is the “foreign” system connected by the gateway.
• Between an organization and an external mail system Your users need the ability to send messages from Microsoft Exchange Server to users on another messaging system such as Internet mail. In this case, you need to install a gateway that translates messages and addresses from Microsoft Exchange Server format to the format of the other messaging system.

Note:   The two scenarios are architecturally identical because, in both cases, Microsoft Exchange Server uses a gateway to transfer messages to and from a messaging system that is external to it. In the first scenario, it makes no difference that the external system is part of the same organization as Microsoft Exchange Server, nor that the external system is probably connected over a LAN rather than over the Internet.

The Microsoft Exchange Server SDK provides the tools for creating gateways (including sample code for gateway functions) and a fully functional sample gateway. You can use the code as a basis for your own gateway because the sample gateway supports much of the same functionality that all gateways should support.