Книга: Linux Network Administrator Guide, Second Edition

Message Routing and Delivery

Exim splits up mail delivery into three different tasks: routing, directing, and transporting. There are a number of code modules of each type, and each is separately configurable. Usually a number of different routers, directors, and transports are set up in the configuration file.

Routers resolve remote addresses, determining which host the message should be sent to and which transport should be used. In Internet-connected hosts there is often just one router, which does the resolution by looking up the domain in the DNS. Alternatively, there may be one router that handles addresses destined for hosts on a local LAN, and a second to send any other addresses to a single smart host; for example, an ISP's mail server.

Local addresses are given to the directors, of which there are normally several, to handle aliasing and forwarding as well as identifying local mailboxes. Mailing lists can be handled by aliasing or forwarding directors. If an address gets aliased or forwarded, any generated addresses are handled independently by the routers or directors, as necessary. By far the most common case will be delivery to a mailbox, but messages may also be piped into a command or appended to a file other than the default mailbox.

A transport is responsible for implementing a method of delivery; for example, sending the message over an SMTP connection or adding it to a specific mailbox. Routers and directors select which transport to use for each recipient address. If a transport fails, Exim either generates a bounce message or defers the address for a later retry.

With Exim, you have a lot of freedom in configuring these tasks. For each of them, a number of drivers are available, from which you can choose those you need. You describe them to Exim in different sections of its configuration file. The transports are defined first, followed by the directors, and then the routers. There are no built-in defaults, though Exim is distributed with a default configuration file that covers simple cases. If you want to change Exim's routing policy or modify a transport, it is easiest to start from the default configuration and make changes rather than attempt to set up a complete configuration from scratch.

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