When a director or a router knows what to do with a particular address, it can send a copy of the message for that address to a named transport. Take, for example, ordinary local delivery. A transport to handle this could be:
local_delivery:
driver = appendfile
file = ${home}/inbox
This is referenced by a director that recognizes the local part of an address as the name of a local user:
localuser: driver = localuser transport = local_delivery
The name `local_delivery' is used in the director to refer to the transport that is to be used. However, there are some transports that are not directly referenced from directors or routers in this way. The need for them arises because aliasing and forwarding operations may be permitted by the configuration to generate destinations such as file names and pipe commands which are not mail addresses. There is no further directing or routing required for these -- they simply have to be associated with an appropriate transport.
The directors that set up such deliveries have private options for specifying which transports to use for these special cases, but if these are not set, the association is done by using conventional names for the transports, as follows:
Examples of these default transports can be seen in the default configuration file. The names used can in fact be changed by means of general configuration options with names `address_file_transport' etc.
For pipe and file names generated by a system filter file (see chapter "System-wide message filtering"), transports can be set explicitly by a number of options with names beginning `message_filter'. If these are not set, the defaults described above are used.
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