Transport providers handle message transmission and reception; they control the interaction between the MAPI spooler and the underlying messaging system and implement security if necessary. They also take care of any preprocessing and postprocessing tasks that are necessary. There is typically one transport provider for every active messaging system.
Client applications communicate with the transport provider through a message store provider. The message store and transport provider communicate through the MAPI spooler. When an incoming message is detected, the transport provider informs the MAPI spooler and the message is delivered to the appropriate message store. To handle outgoing messages, the processing happens in reverse: the message store provider moves the message to the outbound queue, informs the MAPI spooler, and the MAPI spooler transfers it to the appropriate transport provider. When possible, calls to the transport provider are made when client applications are idle. Transport providers and the MAPI spooler operate in the background except at logon time and when prompted by the client application to flush the transmit and receive queues.
Transport providers register with MAPI to handle one or more particular types of recipient entries. When a message is ready to be sent, the MAPI spooler looks at each recipient and determines which transport provider should handle the transmission. Depending on the type of recipient, the MAPI spooler can even call upon more than one transport provider. If the MAPI spooler's first choice is unavailable, and another transport provider has also registered to handle the specific recipient type, the MAPI spooler sends the message to the alternate provider. If the unavailable transport provider is the only one that can handle the recipient, there is no choice except to wait until a connection with that provider can be reestablished.
Some messaging systems are secure systems; all potential users are required to enter a set of valid credentials before access is permitted. MAPI prevents unauthorized access to such secure messaging systems by having the transport provider validate credentials at logon time.