The term transport refers to the mechanism used to deliver a piece of email. Specifically, SMTP and UUCP are mail transports that are supported by Postfix. Transport mapping can be used for a number of purposes, including SMTP to UUCP gatewaying, operating Postfix on a firewall with forwarding to an internal mail server, etc.
This option configures the path to a file containing one or more transport mappings. These tables are much like the mapping tables discussed already, and are converted to a database and used by Postfix in the same way. This option correlates to the transport_maps directive. This feature is disabled by default. A common value for this option is /etc/postfix/trasnport.
To create a new mapping, first define the mapping file. Then click Add a mapping. If your goal is to redirect mail to an protected internal host from Postfix running on a firewall, for example, you could enter the outside domain name into the Name field, swelltech.com and then enter into the Maps to... field the address of the internal machine, smtp:privatehost.swelltech.com. To further improve upon this, local delivery on this machine could be disabled, and increased controls over where and to whom mail should be accepted. There are more examples of such a configuration in the tutorial section of this chapter.