Addressing is an important element of a Microsoft Exchange Server gateway because the gateway must accurately address mail to users in both the Microsoft Exchange Server system and the foreign system it serves. The two primary addressing issues to consider are determining proxy addresses and mapping addresses.
Most foreign systems use an address type and format different from Microsoft Exchange Server. A proxy address is the address by which a Microsoft Exchange Server user is recognized in a foreign mail system. Each Microsoft Exchange Server user can have several proxy addresses—one or more for each foreign system to which Microsoft Exchange Server is connected by a gateway. Also, users can have a primary proxy address, the primary address for a user sending mail to a foreign system, and a secondary proxy address, the address used when receiving mail from the foreign system.
For messages going out from Microsoft Exchange Server, your gateway must convert all addresses to a form that is compatible with the foreign system. For messages coming into Microsoft Exchange Server, it must change all addresses to a form compatible with MAPI and the underlying Microsoft Exchange Server system.