Types of Transport Providers

All transport providers support a range of standard features, such as:

Providing proper security for the underlying messaging system.
Sending and receiving messages from the underlying messaging system.
Exposing the address types the transport providers support so the MAPI spooler and client applications can use them appropriately.
Accepting delivery for specific recipients.

In addition, MAPI supports two specialized types of providers for specific messaging systems.

Transport type	Added functionality
Remote Transport	Enables interoperabilty with remotely connected clients.
TNEF Transport	Allows MAPI properties to be preserved on messaging systems that do not support them.

Remote transports are used by messaging clients that are using low-bandwidth network connections. The most common example is a client operating over a modem link. Remote transports implement the IMAPIFolder : IMAPIContainer interface in addition to the basic functionality implemented by all transports. The IMAPIFolder interface makes it possible for client applications to selectively download messages from a remote message server based on header information at a user's request. Remote transports make themselves known to clients as remote transports by registering themselves in the MAPI status table. For information on implementing remote transports, see Developing a Remote Transport Provider.

TNEF transports are used for translating messages between messaging systems that support different sets of MAPI properties. TNEF transports use the MAPI ITnef : IUnknown interface to convert any properties that the destination system cannot represent directly into a binary data stream that can be attached to the message. Later, another TNEF transport can use ITnef to decode the data stream and make the original MAPI properties available to client applications. Additionally, TNEF support is required if your transport needs to support custom message classes. For information on implementing TNEF transports, see Developing a TNEF-Enabled Transport Provider.

If your transport provider is not one of these types, you will have to implement it with the basic MAPI functions and networking functions available on your target platform.