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, HI-OSF (Hitachi), HP-UX, IRIX, Linux, MIPS RISCOS, NetBSD, OpenBSD, DEC's OSF1 (aka Digital UNIX), SCO, SCO SVR4.2 (aka UNIX-SV), SunOS4, SunOS5, Ultrix, and Unixware. However, code is not available for determining system load averages for AIX or 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'.

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 suggesting and implementing the scheme for building Exim for multiple architectures and operating systems, for porting Exim to several different versions of Unix, and for numerous suggestions. 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 edition of the Exim specification applies to version 1.90 of Exim. Substantive changes from the 1.80 edition are marked by bars in the right margin, except in the Texinfo version of the documentation, because Texinfo doesn't support change bars. Minor corrections and rewordings are not so 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 there is a reasonable batch of changes to make. However, all changes are noted briefly in the distributed file called `doc/ChangeLog', and specifications of new features that are not in this manual are placed in `doc/NewStuff'.

1.1 Mailing list and Web site

`exim-users@exim.org'              general discussion list
`exim-users-digest@exim.org'       digest form of `exim-users'
`exim-announce@exim.org'     moderated, low volume announcements list

The list is also forwarded to `http://www.findmail.com/listserver/exim-users', which is an archiving system with searching capabilities.

1.2 Availability

The current release of Exim is always to be found in

`ftp://ftp.cus.cam.ac.uk/pub/software/programs/exim/exim-n.nn.tar.gz'

where n.nn is the highest such version number in the directory. 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

`exim-patch-n.nn-m.mm.gz'

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

`ftp://ftp.cus.cam.ac.uk/pub/software/programs/exim/exim-pdf-n.nn.tar.gz'

`ftp://ftp.cus.cam.ac.uk/pub/software/programs/exim/exim-postscript-n.nn.tar.gz'
`ftp://ftp.cus.cam.ac.uk/pub/software/programs/exim/exim-texinfo-n.nn.tar.gz'

These tar files contain only the `/doc' directory, not the complete distribution. The documentation is also available online at the web site.

`ftp://ftp.cus.cam.ac.uk/pub/software/programs/exim/Contrib/'

which contains miscellaneous files contributed to the Exim community by Exim users.

There are a number of sites that maintain mirrors of the Exim ftp directory. Those that I know about are listed in the file called `Mirrors'.

1.3 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 fourteen 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, including experimental support for IPv6). However, a pipe transport is available, and there are facilities for writing messages to files and pipes, optionally in batched SMTP format; these facilities can be used to send messages to some other transport mechanism such as UUCP. Batched SMTP input is also catered for.

1.4 Features

These are some of the main features of Exim:

• Exim follows the same general approach of decentralized control that Smail does. There is no central process doing overall management of mail delivery. However, unlike Smail, the independent delivery processes share data in the form of `hints', which makes delivery more efficient in some cases.
• Exim has flexible retry algorithms, applicable to directing and routing addresses as well as to delivery.
• Exim contains header and envelope rewriting facilities.
• Unqualified addresses are accepted only from specified hosts or networks.
• Exim can perform multiple deliveries down the same SMTP channel after deliveries have been delayed.
• Exim can be configured to do local deliveries immediately but to leave remote (SMTP) 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.
• Remote deliveries of the same message to different hosts can optionally be done in parallel.
• Incoming SMTP messages start delivery as soon as they are received, without waiting for the SMTP call to close.
• Perl-compatible regular expressions are available in a number of configuration parameters.
• Domain lists can include file lookups, making it possible to support very large numbers of local domains.
• Exim supports optional checking of incoming return path (sender) and receiver addresses as they are received by 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. Exim also supports the use of the Realtime Blocking List (RBL).
• Hosts that are permitted to relay mail through a machine to another external domain can be controlled by IP number or IP network number. Relay control by recipient domain and sender address is also available.
• Messages on the queue can be `frozen' and `thawed' by the administrator.
• Exim can handle a number of independent local domains on the same machine; each domain can have its own alias files, etc. This facility is sometimes known as `virtual domains'.
• Exim stats a user's home directory before looking for a `.forward' file, in order to detect the case of a missing NFS mount. Delivery is delayed if the directory is unavailable.
• Exim contains an optional built-in mail filtering facility. This can be configured to allow users to provide personal filter files, and it is also possible for a system-wide filter file to be applied to every message.
• 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. There is support for the ETRN command in SMTP to interface to this.
• 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. See chapter "Security considerations" for a discussion of security issues.
• I have tried to make the wording of delivery failure messages clearer and simpler, for the benefit of those less-experienced people who are now using email. Alternative wording for these messages can be provided in a separate file.
• 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. However, all such actions are also available from the command line interface.

1.5 Support for IPv6

The current IP protocol is more strictly called IPv4. IPv6 is the next generation of IP protocol; it is currently in an experimental state. A number of vendors have released IPv6 versions of their systems and 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. Currently, Exim cannot discover for itself the addresses of the host's IPv6 interfaces (at least, not on Solaris 2 -- I don't know the position on other systems) so the `local_interfaces' option must always be used to list them explicitly, in order to prevent mail looping.

See the file `README.IPV6' for general information about the current state of IPv6 support in Exim.

1.6 Interface

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

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 configuration interface is via 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.

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; 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, whose appearance normally causes the message to be transmitted to some other host.

The qualitative 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 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.

Go to the first, previous, next, last section, table of contents.