A gateway's primary task is to transfer messages. Microsoft Exchange Server facilitates this function by wrapping the message in a Message Transfer Envelope (MTE) that provides a list of recipients to whom the gateway must deliver the message. Your gateway must process MTE information and deliver the attached messages, and also transfer several types of reports that inform message originators and recipients about message status.
A gateway must translate messages and transfer their contents between a MAPI mail system format and a foreign system format with minimal loss of information. To accomplish this, each message is translated by a program called a conversion. For each message, the conversion engine selects the correct conversion based on the message class and the gateway.