Introduction to exim

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. It is freely available under the terms of the GNU General Public Licence. In style it is similar to Smail 3, but its facilities are more extensive, and in particular it has some defences against mail bombs and unsolicited junk mail in the form of options for refusing messages from particular hosts, networks, or senders.

The major features are summarised below. There is also an overview which expands on these features. Exim has an extensive set of documentation included in the exim specification, additionally there are documents on the filtering functionality and other documentation is being made available on this web site. The full overview is also available.

Basic Features

Many configuration options can be given as expansion strings, and as these can include file lookups, much of Exim's operation can be made table-driven if desired. For example, it is possible to do local delivery on a machine on which the users do not have accounts.
Regular expressions are available in a number of configuration parameters.
Domain lists can include file lookups, making it possible to support a large number of local domains.
The maximum size of message can be specified.
Exim can handle a number of independent local domains on the same machine; each domain can have its own alias files, etc. These are commonly called virtual domains.
Exim contains an optional built-in mail filtering facility. This enables users to set up their own mail filtering in a straightforward manner without the need to run an external program. There can also be a system filter file that applies to all messages.
Periodic warnings are automatically sent to messages' senders when delivery is delayed -- the time between warnings is configurable.
A queue run can be manually started to deliver just a particular portion of the queue, or those messages with a recipient whose address contains a given string.
Exim can be configured to run as root all the time, except when performing local deliveries, which it always does in a separate process under an appropriate uid and gid. Alternatively, it can be configured to run as root only when needed; in particular, it need not run as root when receiving incoming messages or when sending out messages over SMTP.

Incoming SMTP

SMTP calls from specific machines, optionally from specific idents, can be locked out, and incoming SMTP messages from specific senders can also be locked out.
Messages on the queue can be `frozen' and `thawed' by the administrator.

Outgoing SMTP

Exim can perform multiple deliveries down the same SMTP channel after deliveries to a host have been delayed.
Exim can be configured to do local deliveries immediately but to leave remote deliveries until the message is picked up by a queue-runner process. This increases the likelihood of multiple messages being sent down a single SMTP connection.
When copies of a message have to be delivered to more than one remote host, up to a configured maximum number of remote deliveries can be done in parallel.

Local Deliveries

Exim stats a user's home directory before looking for a `.forward' file, in order to detect the case of a missing NFS mount.
There is support for multiple user mailboxes controlled by prefixes or suffixes on the user name, either via the filter mechanism or through multiple `.forward' files.

Monitoring and Performance Tools

The Exim Monitor is an optional extra; it displays information about Exim's processing in an X window, and an administrator can perform a number of control actions from the window interface.
There are a set of tools for summarising the queue, determining what each exim process is currently doing, examining the deliveries hints databases, and summarising the log files into a concise report on activity.

SPAM/UCE/UBE Limitation Features

Senders can be blocked using a variety of methods, including the ability to apply a set of username or regular expression patterns to incoming mail from particular (or all) domains.
All addresses can be checked for validity during the SMTP transaction thus allowing the elimination of mail from non-existent domains.
Relaying can be tightly controlled based on sending host, network or sending/recipient domains.
If all else fails the system filter can be used to deal with messages based on arbitrary conditions that you can program in (for example block mail with a particular X-Mailer: header.

Limitations

Exim is written in ANSI C. This should not be much of a limitation these days. However, to help with systems that lack a true ANSI C library, Exim avoids making any use of the value returned by the `sprintf()' function, which is one of the main incompatibilities. It has its own version of `strerror()' for use with SunOS4 and any other system that lacks this function, and a macro can be defined to turn `memmove()' into `bcopy()' if necessary.
Exim uses file names that are longer than 14 characters.
Exim is intended for use as an Internet mailer, and therefore handles addresses in RFC 822 domain format only. It cannot handle bang paths, though simple two-component bang paths can be converted by a straightforward rewriting configuration.
Exim insists that every address it handles has a domain attached. For incoming local messages, domainless addresses are automatically qualified with a configured domain value. Configuration options specify from which remote systems unqualified addresses are acceptable.
The only external transport currently implemented is an SMTP transport over a TCP/IP network (using sockets), suitable for machines on the Internet. However, a pipe transport is available, and there are facilities for writing messages to files in `batched SMTP' format; this can be used to send messages to some other transport mechanism. Batched SMTP input is also catered for.

Nigel Metheringham