This chapter gives you a quick introduction to setting up Exim and an overview of its functionality. Although Exim is largely compatible with sendmail in its behavior, its configuration files are completely different.
The main configuration file is usually called /etc/exim.conf or /etc/exim/config in most Linux distributions, or /usr/lib/exim/config in older configurations. You can find out where the configuration file is by running the command:
$ exim -bP configure_file
You may have to edit the configuration file to reflect values specific to your site. In most common configurations there isn't a great deal to change, and a working configuration should rarely have to be modified.
By default, Exim processes and delivers all incoming mail immediately. If you have relatively high traffic, you may instead have Exim collect all messages in the so-called queue, and process them at regular intervals only.
When handling mail within a TCP/IP network, Exim is frequently run in daemon mode: at system boot time, it is invoked from /etc/init.d/exim and puts itself in the background, where it waits for incoming TCP connections on the SMTP port (usually port 25). This is beneficial whenever you expect to have a significant amount of traffic because Exim doesn't have to start up for every incoming connection. Alternatively, inetd could manage the SMTP port and have it spawn Exim whenever there is a connection on this port. This configuration might be useful when you have limited memory and low mail traffic volumes.
Exim has a complicated set of command-line options, including many that match those of sendmail. Instead of trying to put together exactly the right options for your needs, you can implement the most common types of operation by invoking traditional commands like rmail or rsmtp. These are symbolic links to Exim (or if they're not, you can easily link them to it). When you run one of the commands, Exim checks the name you used to invoke it and sets the proper options itself.
There are two links to Exim that you should have under all circumstances: /usr/bin/rmail and /usr/sbin/sendmail. When you compose and send a mail message with a user agent like elm, the message is piped to sendmail or rmail for delivery, which is why both /usr/sbin/sendmail and /usr/bin/rmail should point to Exim. The list of recipients for the message is passed to Exim on the command line. The same happens with mail coming in via UUCP. You can set up the required pathnames to point to Exim by typing the following at a shell prompt:
$ ln -s /usr/sbin/exim /usr/bin/rmail $ ln -s /usr/sbin/exim /usr/sbin/sendmail
If you want to dig further into the details of configuring Exim, you should consult the full Exim specification. If this isn't included in your favorite Linux distribution, you can get it from the source to Exim, or read it online from Exim's web site at http://www.exim.org.
Other possible locations are /etc/rc.d/init.d and rc.inet2. The latter is common on systems using a BSD-style structure for system administration files in the /etc directory.
This is the new standard location of sendmail according to the Linux File System Standard. Another common location is /usr/lib/sendmail, which is likely to be used by mail programs that are not specially configured for Linux. You can define both filenames as symbolic links to Exim so that programs and scripts invoking sendmail will instead invoke Exim to do the same things.
Some user agents, however, use the SMTP protocol to pass messages to the transport agent, calling it with the –bs option.