Olaf Kirch - Linux Network Administrator Guide, Second Edition

When given an address to deliver, Exim first checks whether the domain is one that is handled on the local host by matching it against a list in the local_domains configuration variable. If this option is not set, the local host name is used as the only local domain. If the domain is local, the address is handed to the directors. Otherwise, it is handed to the routers to find out which host to forward a message to. [118] This is a simplification. It is possible for directors to pass addresses to transports that deliver to remote hosts, and similarly, it is possible for routers to pass addresses to local transports that write the messsage to a file or a pipe. It is also possible for routers to pass addresses to the directors in some circumstances.

Most commonly, a local address is just a user's login name, in which case the message is delivered to the user's mailbox, /var/spool/mail / user-name . Other cases include aliases, mailing list names, and mail forwarding by the user. In these cases, the local address expands to a new list of addresses, which may be either local or remote.

Apart from these "normal" addresses, Exim can handle other types of local message destinations, like filenames and pipe commands. When delivering to a file, Exim appends the message, creating the file if necessary. File and pipe destinations are not addresses in their own right, so you can't send mail to, say, /etc/passwd@vbrew.com and expect to overwrite the password file; deliveries to a specific file are valid only if they come from forwarding or alias files. Note, however, that /etc/passwd@vbrew.com is a syntactically valid email address, but if Exim received it, it would (typically) search for a user whose login name was /etc/passwd, fail to find one, and bounce the message.

In an alias list or forwarding file, a filename is anything that begins with a slash (/) that does not parse as a fully qualified email address. For example, /tmp/junk in a forwarding or alias file is interpreted as a file name, but /tmp/junk@vbrew.com is an email address, though it is not likely to be a very useful one. However, valid addresses of this type are seen when sending mail through X.400 gateways, because X.400 addresses start with a slash.

Similarly, a pipe command may be any Unix command preceded by the pipe symbol (|), unless the string parses as a valid email address complete with domain. Unless you have changed the configuration, Exim does not use a shell to run the command; instead, it splits it up into a command name, arguments itself, and runs it directly. The message is fed to the command on its standard input.

For example, to gate a mailing list into a local newsgroup, you might use a shell script named gateit, and set up a local alias that delivers all messages from this mailing list to the script using |gateit. If the command line contains a comma, it and the preceding pipe symbol must be enclosed in double quotes.

A local address most commonly denotes a user's mailbox. This is normally located in /var/spool/mail and has the name of the user, who also owns the file. If it does not exist, it is created by Exim.

In some configurations, the group is set to the user's group and the mode is 0600. In these cases, delivery processes are run as the user, and the user may delete the mailbox entirely. In other configurations, the mailbox's group is mail , and it has mode 660; delivery processes are run under a system uid and group mail , and users cannot delete their mailbox files, though they can empty them.

Note that although /var/spool/mail is currently the standard place to put the mailbox files, some mail software may be compiled to use different paths, for example, /usr/spool/mail . If delivery to users on your machine fails consistently, you should see if it helps to make this a symbolic link to /var/spool/mail .

The addresses MAILER-DAEMON and postmaster should normally appear in your alias file, expanding into the email address of the system administrator. MAILER-DAEMON is used by Exim as the sender address in bounce messages. It is also recommended that root be set up as an alias for an administrator, especially when deliveries are being run under the permissions of the recipient users, in order to avoid running any delivery as root .

Users can redirect their mail to alternative addresses by creating a .forward file in their home directories. This contains a list of recipients separated by commas and/or newlines. All lines of the file are read and interpreted. Any type of address may be used. A practical example of a .forward file for vacations might be:

janet, "|vacation"

In other descriptions of .forward files, you might see the username at the start preceded by a backslash. This was necessary in some older MTAs to stop a search for a .forward for the new name, which could lead to looping. The backslash is not necessary in Exim, which automatically avoids loops of this kind. [119] A director is skipped if the address it is about to process is one that it has previously processed in the course of generating the present address. However, a backslash is permitted, and in fact it does make a difference in configurations where several domains are being handled at once. Without a backslash, an unqualified username is qualified with a default domain; with a backslash the incoming domain is preserved.

The first address in the forward file delivers the incoming message to janet's mailbox, while the vacation command returns a short notification to the sender. [120] Please, if you choose to use a vacation program, make sure it will not reply to messages sent from mailing lists! It is very annoying to discover that someone has gone on vacation and find a vacation message for every message they've received. Mailing list administrators: this is a good example of why it is bad practice to force the Reply-To: field of mailing list messages to that of the list submission address.

In addition to supporting "traditional" forwarding files, Exim can be configured to allow more complex files called filters . Instead of being just a list of forwarding addresses, a filter file can contain tests on the contents of the incoming message so that, for example, messages could be forwarded only if the subject contained the message "urgent." The system administrator must decide whether to allow users this flexibility.

Exim is able to handle alias files compatible with Berkeley's sendmail alias files. Entries in the alias file can have the following form:

alias : recipients

recipients is a comma-separated list of addresses that will be substituted for the alias. The recipient list may be continued across newlines if the next line begins with whitespace.

A special feature allows Exim to handle mailing lists that are held separately from the alias file: if you specify:include: filename as a recipient, Exim reads the specified file and substitutes its contents as a list of recipients. An alternative to handling mailing lists is shown later in this chapter in "Mailing Lists".

The main aliases file is /etc/aliases . If you make this file world-writable or group-writeable, Exim will refuse to use it and will defer local deliveries. You can control the test it applies to the file's permissions by setting modemask in the system_aliases director.