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1. Introduction

If I have seen further it is by standing on the shoulders of giants.
(Isaac Newton).

Exim is a mail transfer agent (MTA) for Unix systems connected to the Internet. Configuration files currently exist for the following operating systems: AIX, BSDI, DGUX, FreeBSD, GNU/Hurd, GNU/Linux, HI-OSF (Hitachi), HP-UX, IRIX, MIPS RISCOS, NetBSD, OpenBSD, QNX, SCO, SCO SVR4.2 (aka UNIX-SV), Solaris (aka SunOS5), SunOS4, Tru64-Unix (formerly Digital UNIX, formerly DEC-OSF1), Ultrix, and Unixware. However, code is not available for determining system load averages under Ultrix.

The terms and conditions for the use and distribution of Exim are contained in the file `NOTICE'. Exim is distributed under the terms of the GNU General Public Licence, a copy of which may be found in the file `LICENCE'.

The use, supply or promotion of Exim for the purpose of sending bulk, unsolicited electronic mail is incompatible with the basic aims of the program, which revolve around the free provision of a service that enhances the quality of personal communications. The author of Exim regards indiscriminate mass-mailing as an antisocial, irresponsible abuse of the Internet.

Exim owes a great deal to Smail 3 and its author, Ron Karr. Without the experience of running and working on the Smail 3 code, I could never have contemplated starting to write a new mailer. Many of the ideas and user interfaces are taken from Smail 3, though the actual code of Exim is entirely new.

I am indebted to my colleague Piete Brooks for originally implementing the scheme for building Exim for multiple architectures and operating systems, for porting early versions of Exim to several different versions of Unix, and for numerous suggestions when I was first developing it. Many other people, both in Cambridge and around the world, have contributed to the development and the testing of Exim, and to porting it to various operating systems. I am grateful to them all.

This document is very much a reference manual; it is not a tutorial. 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-user, special-purpose features that are unlikely to be of very wide interest. It is hoped that, within the next year, a book about Exim will be published which will provide more in-depth explanatory, introductory, and tutorial material.

This edition of the Exim specification applies to version 3.20 of Exim. Substantive changes from the 3.10 edition are marked by bars in the right-hand margin in the PostScript, PDF, and plain text versions of the document. Changes are not marked in the Texinfo version, because Texinfo doesn't support change bars. In the HTML version, a different colour is used. Minor corrections and rewordings are not marked.

As the program is still developing, there may be features in later versions of the program 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. However, all changes are noted briefly in the file called `doc/ChangeLog', and specifications of new features that are not yet in this manual are placed in `doc/NewStuff'. Complete lists of options are maintained in `doc/OptionsLists.txt'.

1.1 Web site and Mailing list

There is a web site at http://www.exim.org by courtesy of Planet Online Ltd, who are situated in the UK. The site is mirrored in the USA and a number of of other countries; links to the mirrors are listed on the home page. Planet Online also run the following mailing lists:

exim-users@exim.org              general discussion list
exim-announce@exim.org           moderated, low volume announcements list
pop-imap@exim.org                discussion of POP/IMAP issues

You can subscribe to these lists, change your existing subscription, and view or search the archives via the `mailing lists' link on the Exim home page.

By courtesy of Martin Hamilton, there is also an archive of the exim-users list in plain text form at http://www.roads.lut.ac.uk/lists/exim-users/exim-users.archive and in HTML via Hypermail at http://www.roads.lut.ac.uk/lists/exim-users/. The list is also forwarded to http://www.egroups.com/list/exim-users, which is another archiving system with searching capabilities.

1.2 Availability

The master ftp site for the Exim distribution is


Those mirror sites that I know about are listed in the file


The current release of Exim is always to 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. When there is only a small amount of change from one version to the next, a patch file may be provided, with a final component name of the form


For each released version, the log of changes is made separately available in the directory


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:


These tar files contain only the `/doc' directory, not the complete distribution, and are also available in `.bz2' as well as `.gz' forms.

An FAQ is available in two different formats from


The FAQ and other HTML documentation is also available online at the web site and its mirrors.

At the ftp site, there is a directory called


which contains miscellaneous files contributed to the Exim community by Exim users, and there is also a collection of contributed configuration examples in


1.3 Limitations

1.4 Features

These are some of the main features of Exim:

1.5 Support for IPv6

IPv6 is the next generation of IP protocol which will in time replace IPv4; it is currently in an experimental state. A number of vendors have already released IPv6 versions of their systems and networking libraries.

If Exim is built with HAVE_IPV6 set, it uses the IPv6 API for TCP/IP input and output. IP addresses can be given in IPv6 as well as IPv4 notation; incoming IPv4 calls use the embedded IPv6 address notation. In the DNS, two new record types, A6 and AAAA, are used for finding IPv6 addresses. A6 records are supposed, in time, to supersede AAAA records. At present, to be on the safe side, when trying to find host addresses from the DNS, Exim looks for all three record types: A6, AAAA, and A, in that order, and builds a combined list of addresses found (dropping any duplicates). In future this may change (for example, to stop once one kind of address has been found).

1.6 Calling interface

Like many MTAs, Exim has adopted the Sendmail interface so that it can be a straight replacement for `/usr/lib/sendmail' or `/usr/sbin/sendmail'. All the relevant Sendmail options are implemented, with two reservations. There are also some additional options that are compatible with Smail 3, and some further options that are new to Exim.

The -t option, for taking a list of recipients from a message's headers, is documented (for several versions of Sendmail) as suppressing delivery to any addresses on the command line (see `man' pages on a number of operating systems). However, it appears that this is not the case in practice. For this reason, Exim has an option called extract_addresses_remove_arguments which controls its behaviour in this regard.

Sendmail uses the -bi option as a request to rebuild the alias file. As Exim does not have the concept of a single alias file, it cannot mimic this behaviour. It can be configured to run a particular script when this option is received; otherwise the option is ignored.

The run time configuration is held in a single text file which 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 installations is provided in the 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 contains a menu interface to Exim's command line administration options.

1.7 Terminology

The term local part, which is taken from RFC 822, 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 word domain is sometimes used to mean all but the first component of a machine's name. It is not used in that sense here, where it normally refers to the part of an email address following the @ sign.

Local domains are mail domains for which the current host is responsible for handling the entire address; in other words, it has special knowledge of what to do with messages sent to such domains, and normally that means using the local part of the address either to deliver the message on the local host or to transform the address using an alias file or something similar. All other domains are remote domains, which normally cause the message to be transmitted to some other host.

The distinction between local and remote domains is not always entirely clear-cut, since a host can have special knowledge about routing for remote domains, and messages for local domains may under some circumstances be passed to other hosts.

The terms local delivery and remote delivery are used to distinguish delivery to a file or a pipe on the local machine from delivery by SMTP to some remote machine. The type of delivery does not necessarily correspond to the type of address. Mail for a local domain may get passed on to some other host, while mail for a remote domain might get delivered locally to a file or pipe for onward transmission by some other means. However, these are special cases.

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 term mailmaster is used to refer to the person in charge of maintaining the mail software on a given computer. Commonly this will be the same person who fulfils the postmaster role, but this may not always be the case.

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.

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