This document is very much a reference manual; it is not a tutorial. The reader is expected to have some familiarity with the SMTP mail transfer protocol and with general Unix system administration. Although there are some discussions and examples in places, the information is mostly organized in a way that makes it easy to look up, rather than in a natural order for sequential reading. Furthermore, the manual aims to cover every aspect of Exim in detail, including a number of rarely-used, special-purpose features that are unlikely to be of very wide interest.

An “easier” discussion of Exim which provides more in-depth explanatory, introductory, and tutorial material can be found in a book entitled The Exim SMTP Mail Server, published by UIT Cambridge (http://www.uit.co.uk/exim-book/).

This book also contains a chapter that gives a general introduction to SMTP and Internet mail. Inevitably, however, the book is unlikely to be fully up-to-date with the latest release of Exim. (Note that the earlier book about Exim, published by O’Reilly, covers Exim 3, and many things have changed in Exim 4.)

The command man update-exim.conf is another source of Debian-specific information.

As the program develops, there may be features in newer versions that have not yet made it into this document, which is updated only when the most significant digit of the fractional part of the version number changes. Specifications of new features that are not yet in this manual are placed in the file doc/NewStuff in the Exim distribution.

Some features may be classified as “experimental”. These may change incompatibly while they are developing, or even be withdrawn. For this reason, they are not documented in this manual. Information about experimental features can be found in the file doc/experimental.txt.

All changes to the program (whether new features, bug fixes, or other kinds of change) are noted briefly in the file called doc/ChangeLog.

This specification itself is available as an ASCII file in doc/spec.txt so that it can easily be searched with a text editor. Other files in the doc directory are:

The main specification and the specification of the filtering language are also available in other formats (HTML, PostScript, PDF, and Texinfo). Section 1.6 below tells you how to get hold of these.

The primary site for Exim source distributions is currently the University of Cambridge’s FTP site, whose contents are described in Where to find the Exim distribution below. In addition, there is a web site and an FTP site at exim.org. These are now also hosted at the University of Cambridge. The exim.org site was previously hosted for a number of years by Energis Squared, formerly Planet Online Ltd, whose support I gratefully acknowledge.

As well as Exim distribution tar files, the Exim web site contains a number of differently formatted versions of the documentation, including the FAQ in both text and HTML formats. The HTML version comes with a keyword-in-context index. A recent addition to the online information is the Exim wiki (http://www.exim.org/eximwiki/). We hope that this will make it easier for Exim users to contribute examples, tips, and know-how for the benefit of others.

The following are the three main Exim mailing lists:

You can subscribe to these lists, change your existing subscriptions, and view or search the archives via the mailing lists link on the Exim home page. The exim-users mailing list is also forwarded to http://www.egroups.com/list/exim-users, an archiving system with searching capabilities.

From time to time (approximately annually at the time of writing), training courses are run by the author of Exim in Cambridge, UK. Details can be found on the web site http://www-tus.csx.cam.ac.uk/courses/exim/.

Reports of obvious bugs should be emailed to bugs@exim.org. However, if you are unsure whether some behaviour is a bug or not, the best thing to do is to post a message to the exim-dev mailing list and have it discussed.

The master ftp site for the Exim distribution is

This is mirrored by

The file references that follow are relative to the exim directories at these sites. There are now quite a number of independent mirror sites around the world. Those that I know about are listed in the file called Mirrors.

Within the exim directory there are subdirectories called exim3 (for previous Exim 3 distributions), exim4 (for the latest Exim 4 distributions), and Testing for testing versions. In the exim4 subdirectory, the current release can always be found in files called

where n.nn is the highest such version number in the directory. The two files contain identical data; the only difference is the type of compression. The .bz2 file is usually a lot smaller than the .gz file.

The distributions are currently signed with Philip Hazel’s GPG key. The corresponding public key is available from a number of keyservers, and there is also a copy in the file Public-Key. The signatures for the tar bundles are in:

For each released version, the log of changes is made separately available in a separate file in the directory ChangeLogs so that it is possible to find out what has changed without having to download the entire distribution.

The main distribution contains ASCII versions of this specification and other documentation; other formats of the documents are available in separate files inside the exim4 directory of the FTP site:

These tar files contain only the doc directory, not the complete distribution, and are also available in .bz2 as well as .gz forms. The FAQ is available for downloading in two different formats in these files:

The first of these is a single ASCII file that can be searched with a text editor. The second is a directory of HTML files, normally accessed by starting at index.html. The HTML version of the FAQ (which is also included in the HTML documentation tarbundle) includes a keyword-in-context index, which is often the most convenient way of finding your way around.

A wish list is maintained, containing ideas for new features that have been submitted. From time to time the file is exported to the ftp site into the file exim4/WishList. Items are removed from the list if they get implemented.

At the ftp site, there is a directory called Contrib that contains miscellaneous files contributed to the Exim community by Exim users. There is also a collection of contributed configuration examples in exim4/config.samples.tar.gz. These samples are referenced from the FAQ.

Exim’s run time configuration is held in a single text file that is divided into a number of sections. The entries in this file consist of keywords and values, in the style of Smail 3 configuration files. A default configuration file which is suitable for simple online installations is provided in the distribution, and is described in chapter 7 below.

Like many MTAs, Exim has adopted the Sendmail command line interface so that it can be a straight replacement for /usr/lib/sendmail or /usr/sbin/sendmail when sending mail, but you do not need to know anything about Sendmail in order to run Exim. For actions other than sending messages, Sendmail-compatible options also exist, but those that produce output (for example, -bp, which lists the messages on the queue) do so in Exim’s own format. There are also some additional options that are compatible with Smail 3, and some further options that are new to Exim. Chapter 5 documents all Exim’s command line options. This information is automatically made into the man page that forms part of the Exim distribution.

Control of messages on the queue can be done via certain privileged command line options. There is also an optional monitor program called eximon, which displays current information in an X window, and which contains a menu interface to Exim’s command line administration options.

The body of a message is the actual data that the sender wants to transmit. It is the last part of a message, and is separated from the header (see below) by a blank line.

When a message cannot be delivered, it is normally returned to the sender in a delivery failure message or a “non-delivery report” (NDR). The term bounce is commonly used for this action, and the error reports are often called bounce messages. This is a convenient shorthand for “delivery failure error report”. Such messages have an empty sender address in the message’s envelope (see below) to ensure that they cannot themselves give rise to further bounce messages.

The term default appears frequently in this manual. It is used to qualify a value which is used in the absence of any setting in the configuration. It may also qualify an action which is taken unless a configuration setting specifies otherwise.

The term defer is used when the delivery of a message to a specific destination cannot immediately take place for some reason (a remote host may be down, or a user’s local mailbox may be full). Such deliveries are deferred until a later time.

The word domain is sometimes used to mean all but the first component of a host’s name. It is not used in that sense here, where it normally refers to the part of an email address following the @ sign.

A message in transit has an associated envelope, as well as a header and a body. The envelope contains a sender address (to which bounce messages should be delivered), and any number of recipient addresses. References to the sender or the recipients of a message usually mean the addresses in the envelope. An MTA uses these addresses for delivery, and for returning bounce messages, not the addresses that appear in the header lines.

The header of a message is the first part of a message’s text, consisting of a number of lines, each of which has a name such as From:, To:, Subject:, etc. Long header lines can be split over several text lines by indenting the continuations. The header is separated from the body by a blank line.

The term local part, which is taken from RFC 2822, is used to refer to that part of an email address that precedes the @ sign. The part that follows the @ sign is called the domain or mail domain.

The terms local delivery and remote delivery are used to distinguish delivery to a file or a pipe on the local host from delivery by SMTP over TCP/IP to another host. As far as Exim is concerned, all hosts other than the host it is running on are remote.

Return path is another name that is used for the sender address in a message’s envelope.

The term queue is used to refer to the set of messages awaiting delivery, because this term is in widespread use in the context of MTAs. However, in Exim’s case the reality is more like a pool than a queue, because there is normally no ordering of waiting messages.

The term queue runner is used to describe a process that scans the queue and attempts to deliver those messages whose retry times have come. This term is used by other MTAs, and also relates to the command runq, but in Exim the waiting messages are normally processed in an unpredictable order.

The term spool directory is used for a directory in which Exim keeps the messages on its queue – that is, those that it is in the process of delivering. This should not be confused with the directory in which local mailboxes are stored, which is called a “spool directory” by some people. In the Exim documentation, “spool” is always used in the first sense.