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Filtering with the Exim Mail Transfer Agent
*******************************************

The specifications of the Exim Mail Transfer Agent's filtering facility
is converted mechanically into Texinfo format from its original
marked-up source.  Some typographic representations are changed,
chapters and sections cannot be numbered, and Texinfo lacks the ability
to mark updated parts of the specification with change bars.

Because the chapters and sections are unnumbered, cross references are
set to their names. This makes the English a bit odd, with phrases like
`see section "Multiple personal mailboxes"' but it seemed very
cumbersome to change this to `see the section entitled "Multiple
personal mailboxes"' each time.

* Menu:

* Exim's interface to mail filtering::


File: exim_filter.info,  Node: Exim's interface to mail filtering,  Prev: Top,  Up: Top

Exim's interface to mail filtering
**********************************

Exim is a mail transfer agent for Unix-like systems. This document
describes the user interface to its in-built mail filtering facility,
and is copyright (c) University of Cambridge 2002. It corresponds to
Exim version 4.10.
------------------------------------------------------------------------


* Menu:

* Introduction::
* Testing a new filter file::
* Installing a filter file::
* Testing an installed filter file::
* Format of filter files::
* String expansion::
* Some useful general variables::
* Header variables::
* User variables::
* Significant deliveries::
* Filter commands::
* The add command::
* The deliver command::
* The save command::
* The pipe command::
* Mail commands::
* Logging commands::
* The finish command::
* The testprint command::
* The fail command::
* The freeze command::
* Obeying commands conditionally::
* String testing conditions::
* Numeric testing conditions::
* Testing for personal mail::
* Testing for significant deliveries::
* Testing for error messages::
* Testing delivery status::
* Testing a list of addresses::
* Multiple personal mailboxes::
* Ignoring delivery errors::
* Examples of filter commands::


File: exim_filter.info,  Node: Introduction,  Next: Testing a new filter file,  Prev: Exim's interface to mail filtering,  Up: Exim's interface to mail filtering

Introduction
============

Most Unix mail transfer agents (programs that deliver mail) permit
individual users to specify automatic forwarding of their mail, usually
by placing a list of forwarding addresses in a file called `.forward'
in their home directories.  Exim extends this facility by allowing the
forwarding instructions to be a set of rules rather than just a list of
addresses, in effect providing ``.forward' with conditions'. Operating
the set of rules is called filtering, and the file that contains them
is called a filter file.

The ability to use filtering has to be enabled by the system
administrator, and some of the individual facilities can be separately
enabled or disabled.  A local document should be provided to describe
exactly what has been enabled.  In the absence of this, consult your
system administrator.

It is important to realize that no deliveries are actually made while a
filter file is being processed. The result of filtering is a list of
destinations to which a message should be delivered - the deliveries
themselves take place later, along with all other deliveries for the
message. This means that it is not possible to test for successful
deliveries while filtering. It also means that duplicate addresses
generated by filtering are dropped, as with any other duplicate
addresses.

This document describes how to use a filter file and the format of its
contents. It is intended for use by end-users. How the system
administrator can set up and control the use of filtering is described
in the full Exim specification.


File: exim_filter.info,  Node: Testing a new filter file,  Next: Installing a filter file,  Prev: Introduction,  Up: Exim's interface to mail filtering

Testing a new filter file
=========================

Filter files, especially the more complicated ones, should always be
tested, as it is easy to make mistakes. Exim provides a facility for
preliminary testing of a filter file before installing it. This tests
the syntax of the file and its basic operation, and can also be used
with ordinary `.forward' files.

Because a filter can do tests on the content of messages, a test
message is required. Suppose you have a new filter file called
`myfilter' and a test message called `test-message'. Assuming that Exim
is installed with the conventional path name `/usr/sbin/sendmail' (some
operating systems use `/usr/lib/sendmail'), the following command can
be used:

     /usr/sbin/sendmail -bf myfilter <test-message

The -bf option tells Exim that the following item on the command line
is the name of a filter file which is to be tested. There is also a -bF
option, which is similar, but which is used for testing system filter
files, as opposed to user filter files, and which is therefore of use
only to the system administrator.

The test message is supplied on the standard input. If there are no
message-dependent tests in the filter, an empty file (`/dev/null') can
be used. A supplied message must start with header lines or the `From'
message separator line which is found in many multi-message folder
files. Note that blank lines at the start terminate the header lines. A
warning is given if no header lines are read.

The result of running this command, provided no errors are detected in
the filter file, is a list of the actions that Exim would try to take
if presented with the message for real. For example, the output

     Deliver message to: gulliver@lilliput.fict.example
     Save message to: /home/lemuel/mail/archive

means that one copy of the message would be sent to
"gulliver@lilliput.fict.example", and another would be added to the file
`/home/lemuel/mail/archive', if all went well.

The actions themselves are not attempted while testing a filter file in
this way; there is no check, for example, that any forwarding addresses
are valid.  If you want to know why a particular action is being taken,
add the -v option to the command. This causes Exim to output the
results of any conditional tests and to indent its output according to
the depth of nesting of `if' commands.  Further additional output from
a filter test can be generated by the `testprint' command, which is
described below.

When Exim is outputting a list of the actions it would take, if any text
strings are included in the output, non-printing characters therein are
converted to escape sequences. In particular, if any text string
contains a newline character, this is shown as `\n' in the testing
output.

When testing a filter in this way, Exim makes up an `envelope' for the
message.  The recipient is by default the user running the command, and
so is the sender, but the command can be run with the -f option to
supply a different sender.  For example,

     /usr/sbin/sendmail -bf myfilter -f islington@neverwhere <test-message

Alternatively, if the -f option is not used, but the first line of the
supplied message is a `From' separator from a message folder file (not
the same thing as a `From:' header line), the sender is taken from
there. If -f is present, the contents of any `From' line are ignored.

The `return path' is the same as the envelope sender, unless the message
contains a `Return-path:' header, in which case it is taken from there.
You need not worry about any of this unless you want to test out
features of a filter file that rely on the sender address or the return
path.

It is possible to change the envelope recipient by specifying further
options.  The -bfd option changes the domain of the recipient address,
while the -bfl option changes the `local part', that is, the part
before the @ sign.  An adviser could make use of these to test someone
else's filter file.

The -bfp and -bfs options specify the prefix or suffix for the local
part.  These are relevant only when support for multiple personal
mailboxes is implemented; see the description in section "Multiple
personal mailboxes" below.


File: exim_filter.info,  Node: Installing a filter file,  Next: Testing an installed filter file,  Prev: Testing a new filter file,  Up: Exim's interface to mail filtering

Installing a filter file
========================

A filter file is normally installed under the name `.forward' in your
home directory - it is distinguished from a conventional `.forward'
file by its first line (described below). However, the file name is
configurable, and some system administrators may choose to use some
different name or location for filter files.


File: exim_filter.info,  Node: Testing an installed filter file,  Next: Format of filter files,  Prev: Installing a filter file,  Up: Exim's interface to mail filtering

Testing an installed filter file
================================

Testing a filter file before installation cannot find every potential
problem; for example, it does not actually run commands to which
messages are piped.  Some `live' tests should therefore also be done
once a filter is installed.

If at all possible, test your filter file by sending messages from some
other account. If you send a message to yourself from the filtered
account, and delivery fails, the error message will be sent back to the
same account, which may cause another delivery failure. It won't cause
an infinite sequence of such messages, because delivery failure
messages do not themselves generate further messages. However, it does
mean that the failure won't be returned to you, and also that the
postmaster will have to investigate the stuck message.

If you have to test a filter from the same account, a sensible
precaution is to include the line

     if error_message then finish endif

as the first filter command, at least while testing. This causes
filtering to be abandoned for a delivery failure message, and since no
destinations are generated, the message goes on to get delivered to the
original address. Unless there is a good reason for not doing so, it is
recommended that the above test be left in all filter files.


File: exim_filter.info,  Node: Format of filter files,  Next: String expansion,  Prev: Testing an installed filter file,  Up: Exim's interface to mail filtering

Format of filter files
======================

Apart from leading white space, the first text in a filter file must be

     # Exim filter

This is what distinguishes it from a conventional `.forward' file. If
the file does not have this initial line it is treated as a conventional
`.forward' file, both when delivering mail and when using the -bf
testing mechanism. The white space in the line is optional, and any
capitalization may be used. Further text on the same line is treated as
a comment. For example, you could have

     #   Exim filter   <<== do not edit or remove this line!

The remainder of the file is a sequence of filtering commands, which
consist of keywords and data values. For example, in the command

     deliver gulliver@lilliput.fict.example

the keyword is `deliver' and the data value is
`gulliver@lilliput.fict.example'.  White space or line breaks separate
the components of a command, except in the case of conditions for the
`if' command, where round brackets (parentheses) also act as
separators. Complete commands are separated from each other by white
space or line breaks; there are no special terminators. Thus, several
commands may appear on one line, or one command may be spread over a
number of lines.

If the character # follows a separator anywhere in a command,
everything from # up to the next newline is ignored. This provides a
way of including comments in a filter file.

There are two ways in which a data value can be input:

   * If the text contains no white space then it can be typed verbatim.
     However, if it is part of a condition, it must also be free of
     round brackets (parentheses), as these are used for grouping in
     conditions.

   * Otherwise it must be enclosed in double quotation marks. In this
     case, the character \ (backslash) is treated as an `escape
     character' within the string, causing the following character or
     characters to be treated specially:

          \n       is replaced by a newline
          \r       is replaced by a carriage return
          \t       is replaced by a tab

     Backslash followed by up to three octal digits is replaced by the
     character specified by those digits, and \x followed by up to two
     hexadecimal digits is treated similarly. Backslash followed by any
     other character is replaced by the second character, so that in
     particular, \" becomes " and \\ becomes \. A data item enclosed in
     double quotes can be continued onto the next line by ending the
     first line with a backslash. Any leading white space at the start
     of the continuation line is ignored.

In addition to the escape character processing that occurs when strings
are enclosed in quotes, most data values are also subject to string
expansion (as described in the next section), in which case the
characters "$" and "\" are also significant. This means that if a
single backslash is actually required in such a string, and the string
is also quoted, \\\\ has to be entered.


File: exim_filter.info,  Node: String expansion,  Next: Some useful general variables,  Prev: Format of filter files,  Up: Exim's interface to mail filtering

String expansion
================

Most data values are expanded before use. Expansion consists of
replacing substrings beginning with `$' with other text. The full
expansion facilities available in Exim are extensive. If you want to
know everything that Exim can do with strings, you should consult the
chapter on string expansion in the Exim documentation.

In filter files, by far the most common use of string expansion is the
substitution of the contents of a variable. For example, the substring

     $reply_address

is replaced by the address to which replies to the message should be
sent. If such a variable name is followed by a letter or digit or
underscore, it must be enclosed in curly brackets (braces), for example,

     ${reply_address}

If a `$' character is actually required in an expanded string, it must
be escaped with a backslash, and because backslash is also an escape
character in quoted input strings, it must be doubled in that case. The
following two examples illustrate two different ways of testing for a
`$' character in a message:

     if $message_body contains \$ then ...
     if $message_body contains "\\$" then ...

You can prevent part of a string from being expanded by enclosing it
between two occurrences of `\N'. For example,

     if $message_body contains \N$$$$\N then ...

tests for a run of four dollar characters.


File: exim_filter.info,  Node: Some useful general variables,  Next: Header variables,  Prev: String expansion,  Up: Exim's interface to mail filtering

Some useful general variables
=============================

A complete list of the available variables is given in the Exim
documentation.  This shortened list contains the ones that are most
likely to be useful in personal filter files:

$body_linecount: The number of lines in the body of the message.

$home: The user's home directory.

$local_part: The part of the email address that precedes the @ sign -
normally the user's login name. If support for multiple personal
mailboxes is enabled (see section "Multiple personal mailboxes" below)
and a prefix or suffix for the local part was recognized, it is removed
from the string in this variable.

$local_part_prefix: If support for multiple personal mailboxes is
enabled (see section "Multiple personal mailboxes" below), and a local
part prefix was recognized, this variable contains the prefix.
Otherwise it contains an empty string.

$local_part_suffix: If support for multiple personal mailboxes is
enabled (see section "Multiple personal mailboxes" below), and a local
part suffix was recognized, this variable contains the suffix.
Otherwise it contains an empty string.

$message_body: The initial portion of the body of the message. By
default, up to 500 characters are read into this variable, but the
system administrator can configure this to some other value. Newlines
in the body are converted into single spaces.

$message_body_end: The final portion of the body of the message,
formatted and limited in the same way as $message_body.

$message_body_size: The size of the body of the message, in bytes.

$message_headers: The header lines of the message, concatenated into a
single string, with newline characters between them.

$message_id: The message's local identification string, which is unique
for each message handled by a single host.

$message_size: The size of the entire message, in bytes.

$original_local_part: When an address that arrived with the message is
being processed, this contains the same value as the variable
$local_part.  However, if an address generated by an alias, forward, or
filter file is being processed, this variable contains the local part
of the original address.

$reply_address: The contents of the `Reply-to:' header, if the message
has one; otherwise the contents of the `From:' header. It is the
address to which normal replies to the message should be sent.

$return_path: The return path - that is, the sender field that will be
transmitted as part of the message's envelope if the message is sent to
another host. This is the address to which delivery errors are sent. In
many cases, this variable has the same value as $sender_address, but
if, for example, an incoming message to a mailing list has been
expanded, $return_path may have been changed to contain the address of
the list maintainer.

$sender_address: The sender address that was received in the envelope of
the message. This is not necessarily the same as the contents of the
`From:' or `Sender:' header lines. For delivery error messages (`bounce
messages') there is no sender address, and this variable is empty.

$tod_full: A full version of the time and date, for example: Wed, 18 Oct
1995 09:51:40 +0100. The timezone is always given as a numerical offset
from GMT.

$tod_log: The time and date in the format used for writing Exim's log
files, for example: 1995-10-12 15:32:29.


File: exim_filter.info,  Node: Header variables,  Next: User variables,  Prev: Some useful general variables,  Up: Exim's interface to mail filtering

Header variables
================

There is a special set of expansion variables containing the header
lines of the message being processed. These variables have names
beginning with `$header_' followed by the name of the header,
terminated by a colon. For example,

     $header_from:
     $header_subject:

The whole item, including the terminating colon, is replaced by the
contents of the message header line.  If there is more than one header
line with the same name, their contents are concatenated. For header
lines whose data consists of a list of addresses (for example, "From:"
and "To:"), a comma and newline is inserted between each set of data.
For all other header lines, just a newline is used.

The capitalization of the name following `$header_' is not significant.
Because any printing character except colon may appear in the name of a
message's header (this is a requirement of RFC 2822, the document that
describes the format of a mail message) curly brackets must not be used
in this case, as they will be taken as part of the header name. Two
shortcuts are allowed in naming header variables:

   * The initiating `$header_' can be abbreviated to `$h_'.

   * The terminating colon can be omitted if the next character is
     white space. The white space character is retained in the expanded
     string. However, this is not recommended, because it makes it easy
     to forget the colon when it really is needed.

If the message does not contain a header of the given name, an empty
string is substituted. Thus it is important to spell the names of
headers correctly. Do not use `$header_Reply_to' when you really mean
`$header_Reply-to'.


File: exim_filter.info,  Node: User variables,  Next: Significant deliveries,  Prev: Header variables,  Up: Exim's interface to mail filtering

User variables
==============

There are ten user variables with names $n0 - $n9 that can be
incremented by the `add' command (see section "The add command"). These
can be used for `scoring' messages in various ways. If Exim is
configured to run a `system filter' on every message, the values left
in these variables are copied into the variables $sn0 - $sn9 at the end
of the system filter, thus making them available to users' filter
files. How these values are used is entirely up to the individual
installation.


File: exim_filter.info,  Node: Significant deliveries,  Next: Filter commands,  Prev: User variables,  Up: Exim's interface to mail filtering

Significant deliveries
======================

When in the course of delivery a message is processed by a filter file,
what happens next, that is, after the whole filter file has been
processed, depends on whether the filter has set up any significant
deliveries or not. If there is at least one significant delivery, the
filter is considered to have handled the entire delivery arrangements
for the current address, and no further processing of the address takes
place. If, however, no significant deliveries have been set up, Exim
continues processing the current address as if there were no filter
file, and typically sets up a delivery of a copy of the message into a
local mailbox. In particular, this happens in the special case of a
filter file containing only comments.

The delivery commands `deliver', `save', and `pipe' are by default
significant. However, if such a command is preceded by the word
`unseen', its delivery is not considered to be significant. In
contrast, other commands such as `mail' and `vacation' do not count as
significant deliveries unless preceded by the word `seen'.


File: exim_filter.info,  Node: Filter commands,  Next: The add command,  Prev: Significant deliveries,  Up: Exim's interface to mail filtering

Filter commands
===============

The filter commands which are described in subsequent sections are
listed below, with the section in which they are described in brackets:

     "add"              increment a user variable (section "The add command")
     "deliver"          deliver to an email address (section "The deliver command")
     "fail"             force delivery failure (sysadmin use) (section "The fail command")
     "finish"           end processing (section "The finish command")
     "freeze"           freeze message (sysadmin use) (section "The freeze command")
     "if"               test condition(s) (section "Obeying commands conditionally")
     "logfile"          define log file (section "Logging commands")
     "logwrite"         write to log file (section "Logging commands")
     "mail"             send a reply message (section "Mail commands")
     "pipe"             pipe to a command (section "The pipe command")
     "save"             save to a file (section "The save command")
     "testprint"        print while testing (section "The testprint command")
     "vacation"         tailored form of "mail" (section "Mail commands")

In addition, when Exim's filtering facilities are being used as a system
filter, the `fail', `freeze', and `headers' commands are available.
However, since they are usable only by the system administrator and not
by ordinary users, they are described in the main Exim specification
rather than in this document.


File: exim_filter.info,  Node: The add command,  Next: The deliver command,  Prev: Filter commands,  Up: Exim's interface to mail filtering

The add command
===============


          add <number> to <user variable>
     e.g. add 2 to n3

There are 10 user variables of this type, and their values can be
obtained by the normal expansion syntax (for example $n3) in other
commands. At the start of filtering, these variables all contain zero.
Both arguments of the `add' command are expanded before use, making it
possible to add variables to each other. Subtraction can be obtained by
adding negative numbers.


File: exim_filter.info,  Node: The deliver command,  Next: The save command,  Prev: The add command,  Up: Exim's interface to mail filtering

The deliver command
===================


          deliver <mail address>
     e.g. deliver "Dr Livingstone <David@somewhere.africa.example>"

This provides a forwarding operation. The message is sent on to the
given address, exactly as happens if the address had appeared in a
traditional `.forward' file. If you want to deliver the message to a
number of different addresses, you can use more than one `deliver'
command (each one may have only one address). However, duplicate
addresses are discarded.

To deliver a copy of the message to your normal mailbox, your login
name can be given as the address. Once an address has been processed by
the filtering mechanism, an identical generated address will not be so
processed again, so doing this does not cause a loop.

However, if you have a mail alias, you should not refer to it here. For
example, if the mail address `L.Gulliver' is aliased to `lg103' then all
references in Gulliver's `.forward' file should be to `lg103'. A
reference to the alias will not work for messages that are addressed to
that alias, since, like `.forward' file processing, aliasing is
performed only once on an address, in order to avoid looping.

Following the new address, an optional second address, preceded by
`errors_to' may appear. This changes the address to which delivery
errors on the forwarded message will be sent. Instead of going to the
message's original sender, they go to this new address. For ordinary
users, the only value that is permitted for this address is the user
whose filter file is being processed.  For example, the user `lg103'
whose mailbox is in the domain "lilliput.example" could have a filter
file that contains

          deliver jon@elsewhere.example errors_to lg103@lilliput.example

Clearly, using this feature makes sense only in situations where not all
messages are being forwarded. In particular, bounce messages must not be
forwarded in this way, as this is likely to create a mail loop if
something goes wrong.


File: exim_filter.info,  Node: The save command,  Next: The pipe command,  Prev: The deliver command,  Up: Exim's interface to mail filtering

The save command
================


          save <file name>
     e.g. save $home/mail/bookfolder

This causes a copy of the message to be appended to the given file
(that is, the file is used as a mail folder). More than one `save'
command may appear; each one causes a copy of the message to be written
to its argument file, provided they are different (duplicate `save'
commands are ignored).

If the file name does not start with a / character, the contents of the
$home variable are prepended.  In conventional configurations, this
variable is normally set in a user filter.  However, it is never set in
a system filter.

The user must of course have permission to write to the file, and the
writing of the file takes place in a process that is running as the
user, under the user's primary group. Any secondary groups to which the
user may belong are not normally taken into account, though the system
administrator can configure Exim to set them up. In addition, the
ability to use this command at all is controlled by the system
administrator - it may be forbidden on some systems.

An optional mode value may be given after the file name. The value for
the mode is interpreted as an octal number, even if it does not begin
with a zero. For example:

          save /some/folder 640

This makes it possible for users to override the system-wide mode
setting for file deliveries, which is normally 600. If an existing file
does not have the correct mode, it is changed.

An alternative form of delivery may be enabled on your system, in which
each message is delivered into a new file in a given directory. If this
is the case, this functionality can be requested by giving the
directory name terminated by a slash after the `save' command, for
example

         save separated/messages/

There are several different formats for such deliveries; check with
your system administrator or local documentation to find out which (if
any) are available on your system. If this functionality is not
enabled, the use of a path name ending in a slash causes an error.


File: exim_filter.info,  Node: The pipe command,  Next: Mail commands,  Prev: The save command,  Up: Exim's interface to mail filtering

The pipe command
================


          pipe <command>
     e.g. pipe "$home/bin/countmail $sender_address"

This command sets up delivery to a specified command using a pipe.
Remember, however, that no deliveries are done while the filter is
being processed. All deliveries happen later on. Therefore, the result
of running the pipe is not available to the filter.

When the deliveries are done, a separate process is run, and a copy of
the message is passed on its standard input. The process runs as the
user, under the user's primary group. Any secondary groups to which the
user may belong are not normally taken into account, though the system
administrator can configure Exim to set them up. More than one `pipe'
command may appear; each one causes a copy of the message to be written
to its argument pipe, provided they are different (duplicate `pipe'
commands are ignored).

The command supplied to `pipe' is split up by Exim into a command name
and a number of arguments. These are delimited by white space except
for arguments enclosed in double quotes, in which case backslash is
interpreted as an escape, or in single quotes, in which case no
escaping is recognized. Note that as the whole command is normally
supplied in double quotes, a second level of quoting is required for
internal double quotes. For example:

          pipe "$home/myscript \"size is $message_size\""

String expansion is performed on the separate components after the line
has been split up, and the command is then run directly by Exim; it is
not run under a shell. Therefore, substitution cannot change the number
of arguments, nor can quotes, backslashes or other shell metacharacters
in variables cause confusion.

Documentation for some programs that are normally run via this kind of
pipe often suggest that the command should start with

     IFS=" "

This is a shell command, and should not be present in Exim filter files,
since it does not normally run the command under a shell.

However, there is an option that the administrator can set to cause a
shell to be used. In this case, the entire command is expanded as a
single string and passed to the shell for interpretation. It is
recommended that this be avoided if at all possible, since it can lead
to problems when inserted variables contain shell metacharacters.

The default PATH set up for the command is determined by the system
administrator, usually containing at least "/usr/bin" so that common
commands are available without having to specify an absolute file name.
However, it is possible for the system administrator to restrict the
pipe facility so that the command name must not contain any /
characters, and must be found in one of the directories in the
configured PATH. It is also possible for the system administrator to
lock out the use of the `pipe' command altogether.

When the command is run, a number of environment variables are set up.
The complete list for pipe deliveries may be found in the Exim
reference manual.  Those that may be useful for pipe deliveries from
user filter files are:

     DOMAIN                 the domain of the address
     HOME                   your home directory
     LOCAL_PART             see below
     LOCAL_PART_PREFIX      see below
     LOCAL_PART_SUFFIX      see below
     LOGNAME                your login name
     MESSAGE_ID             the message's unique id
     PATH                   the command search path
     RECIPIENT              the complete recipient address
     SENDER                 the sender of the message
     SHELL                  /bin/sh
     USER                   see below

LOCAL_PART, LOGNAME, and USER are all set to the same value, namely,
your login id. LOCAL_PART_PREFIX and LOCAL_PART_SUFFIX may be set if
Exim is configured to recognize prefixes or suffixes in the local parts
of addresses. For example, a message addressed to
"pat-suf2@domain.example" may cause user "pat"'s filter file to be run.
If this sets up a pipe delivery, LOCAL_PART_SUFFIX is `-suf2' when the
pipe command runs. The system administrator has to configure Exim
specially for this feature to be available.

If you run a command that is a shell script, be very careful in your
use of data from the incoming message in the commands in your script.
RFC 2822 is very generous in the characters that are legally permitted
to appear in mail addresses, and in particular, an address may begin
with a vertical bar or a slash. For this reason you should always use
quotes round any arguments that involve data from the message, like
this:

     /some/command '$SENDER'

so that inserted shell meta-characters do not cause unwanted effects.

Remember that, as was explained earlier, the pipe command is not run at
the time the filter file is interpreted. The filter just defines what
deliveries are required for one particular addressee of a message. The
deliveries themselves happen later, once Exim has decided everything
that needs to be done for the message.

A consequence of this is that you cannot inspect the return code from
the pipe command from within the filter. Neverthless, the code returned
by the command is important, because Exim uses it to decide whether the
delivery has succeeded or failed.

The command should return a zero completion code if all has gone well.
Most non-zero codes are treated by Exim as indicating a failure of the
pipe. This is treated as a delivery failure, causing the message to be
returned to its sender. However, there are some completion codes which
are treated as temporary errors. The message remains on Exim's spool
disc, and the delivery is tried again later, though it will ultimately
time out if the delivery failures go on too long. The completion codes
to which this applies can be specified by the system administrator; the
default values are 73 and 75.

The pipe command should not normally write anything to its standard
output or standard error file descriptors. If it does, whatever is
written is normally returned to the sender of the message as a delivery
error, though this action can be varied by the system administrator.


File: exim_filter.info,  Node: Mail commands,  Next: Logging commands,  Prev: The pipe command,  Up: Exim's interface to mail filtering

Mail commands
=============

There are two commands which cause the creation of a new mail message,
neither of which count as a significant delivery unless the command is
preceded by the word `seen'. This is a powerful facility, but it should
be used with care, because of the danger of creating infinite sequences
of messages. The system administrator can forbid the use of these
commands altogether.

To help prevent runaway message sequences, these commands have no
effect when the incoming message is a delivery error message, and
messages sent by this means are treated as if they were reporting
delivery errors. Thus they should never themselves cause a delivery
error message to be returned. The basic mail-sending command is

          mail [to <address-list>]
               [cc <address-list>]
               [bcc <address-list>]
               [from <address>]
               [reply_to <address>]
               [subject <text>]
               [text <text>]
               [[expand] file <filename>]
               [return message]
               [log <log file name>]
               [once <note file name>]
               [once_repeat <time interval>]

     e.g. mail text "Your message about $h_subject: has been received"

As a convenience for use in one common case, there is also a command
called "vacation". It behaves in the same way as "mail", except that
the defaults for the `file', `log', `once', and `once_repeat' options
are

     expand file .vacation.msg
     log  .vacation.log
     once .vacation
     once_repeat 7d

respectively. These are the same file names and repeat period used by
the traditional Unix `vacation' command. The defaults can be overridden
by explicit settings, but if a file name is given its contents are
expanded only if explicitly requested. The `vacation' command is
normally used conditionally, subject to the `personal' condition (see
section "Testing for personal mail" below) so as not to send automatic
replies to non-personal messages from mailing lists or elsewhere.

For both commands, the key/value argument pairs can appear in any
order. At least one of `text' or `file' must appear (except with
`vacation'); if both are present, the text string appears first in the
message. If `expand' precedes `file', each line of the file is subject
to string expansion as it is included in the message.

Several lines of text can be supplied to `text' by including the escape
sequence `\n' in the string where newlines are required. If the command
is output during filter file testing, newlines in the text are shown as
`\n'.

Note that the keyword for creating a `Reply-To:' header is "reply_to",
because Exim keywords may contain underscores, but not hyphens. If the
`from' keyword is present and the given address does not match the user
who owns the forward file, Exim normally adds a `Sender:' header to the
message, though it can be configured not to do this.

If no `to' argument appears, the message is sent to the address in the
`$reply_address' variable (see section "String expansion" above).  An
`In-Reply-To:' header is automatically included in the created message,
giving a reference to the message identification of the incoming
message.

If `return message' is specified, the incoming message that caused the
filter file to be run is added to the end of the message, subject to a
maximum size limitation.

If a log file is specified, a line is added to it for each message sent.

If a `once' file is specified, it is used to hold a database for
remembering who has received a message, and no more than one message is
ever sent to any particular address, unless `once_repeat' is set. This
specifies a time interval after which another copy of the message is
sent. The interval is specified as a sequence of numbers, each followed
by the initial letter of one of `seconds', `minutes', `hours', `days',
or `weeks'. For example,

     once_repeat 5d4h

causes a new message to be sent if 5 days and 4 hours have elapsed
since the last one was sent. There must be no white space in a time
interval.

Commonly, the file name specified for `once' is used as the base name
for direct-access (DBM) file operations. There are a number of
different DBM libraries in existence. Some operating systems provide
one as a default, but even in this case a different one may have been
used when building Exim. With some DBM libraries, specifying `once'
results in two files being created, with the suffixes `.dir' and `.pag'
being added to the given name. With some others a single file with the
suffix `.db' is used, or the name is used unchanged.

Using a DBM file for implementing the `once' feature means that the file
grows as large as necessary. This is not usually a problem, but some
system administrators want to put a limit on it. The facility can be
configured not to use a DBM file, but instead, to use a regular file
with a maximum size. The data in such a file is searched sequentially,
and if the file fills up, the oldest entry is deleted to make way for a
new one. This means that some correspondents may receive a second copy
of the message after an unpredictable interval. Consult your local
information to see if your system is configured this way.

More than one `mail' or `vacation' command may be obeyed in a single
filter run; they are all honoured, even when they are to the same
recipient.


File: exim_filter.info,  Node: Logging commands,  Next: The finish command,  Prev: Mail commands,  Up: Exim's interface to mail filtering

Logging commands
================

A log can be kept of actions taken by a filter file. This facility is
normally available in conventional configurations, but there are some
situations where it might not be. Also, the system administrator may
choose to disable it. Check your local information if in doubt.

Logging takes place while the filter file is being interpreted. It does
not queue up for later like the delivery commands. The reason for this
is so that a log file need be opened only once for several write
operations. There are two commands, neither of which constitutes a
significant delivery. The first defines a file to which logging output
is subsequently written:

          logfile <file name>
     e.g. logfile $home/filter.log

The file name must be fully qualified. You can use $home, as in this
example, to refer to your home directory. The file name may optionally
be followed by a mode for the file, which is used if the file has to be
created.  For example,

          logfile $home/filter.log 0644

The number is interpreted as octal, even if it does not begin with a
zero.  The default for the mode is 600. It is suggested that the
`logfile' command normally appear as the first command in a filter
file. Once `logfile' has been obeyed, the `logwrite' command can be
used to write to the log file:

          logwrite "<some text string>"
     e.g. logwrite "$tod_log $message_id processed"

It is possible to have more than one `logfile' command, to specify
writing to different log files in different circumstances. Writing
takes place at the end of the file, and a newline character is added to
the end of each string if there isn't one already there. Newlines can
be put in the middle of the string by using the `\n' escape sequence.
Lines from simultaneous deliveries may get interleaved in the file, as
there is no interlocking, so you should plan your logging with this in
mind. However, data should not get lost.


File: exim_filter.info,  Node: The finish command,  Next: The testprint command,  Prev: Logging commands,  Up: Exim's interface to mail filtering

The finish command
==================

The command `finish', which has no arguments, causes Exim to stop
interpreting the filter file. This is not a significant action unless
preceded by `seen'. A filter file containing only `seen finish' is a
black hole.


File: exim_filter.info,  Node: The testprint command,  Next: The fail command,  Prev: The finish command,  Up: Exim's interface to mail filtering

The testprint command
=====================

It is sometimes helpful to be able to print out the values of variables
when testing filter files. The command

          testprint <text>
     e.g. testprint "home=$home reply_address=$reply_address"

does nothing when mail is being delivered. However, when the filtering
code is being tested by means of the -bf option (see section "Testing a
new filter file" above), the value of the string is written to the
standard output.


File: exim_filter.info,  Node: The fail command,  Next: The freeze command,  Prev: The testprint command,  Up: Exim's interface to mail filtering

The fail command
================

When Exim's filtering facilities are being used as a system filter, the
`fail' command is available, to force delivery failure. Because this
command is normally usable only by the system administrator, and not
enabled for use by ordinary users, it is described in more detail in
the main Exim specification rather than in this document.


File: exim_filter.info,  Node: The freeze command,  Next: Obeying commands conditionally,  Prev: The fail command,  Up: Exim's interface to mail filtering

The freeze command
==================

When Exim's filtering facilities are being used as a system filter, the
`freeze' command is available, to freeze a message on the queue.
Because this command is normally usable only by the system
administrator, and not enabled for use by ordinary users, it is
described in more detail in the main Exim specification rather than in
this document.


File: exim_filter.info,  Node: Obeying commands conditionally,  Next: String testing conditions,  Prev: The freeze command,  Up: Exim's interface to mail filtering

Obeying commands conditionally
==============================

Most of the power of filtering comes from the ability to test
conditions and obey different commands depending on the outcome. The
`if' command is used to specify conditional execution, and its general
form is

     if    <condition>
     then  <commands>
     elif  <condition>
     then  <commands>
     else  <commands>
     endif

There may be any number of `elif' and `then' sections (including none)
and the `else' section is also optional. Any number of commands,
including nested `if' commands, may appear in any of the <commands>
sections.

Conditions can be combined by using the words `and' and `or', and round
brackets (parentheses) can be used to specify how several conditions
are to combine. Without brackets, `and' is more binding than `or'.  For
example,

     if
       $h_subject: contains "Make money" or
       $h_precedence: is "junk" or
       ($h_sender: matches ^\\d{8}@ and not personal) or
       $message_body contains "this is spam"
     then
       seen finish
     endif

A condition can be preceded by `not' to negate it, and there are also
some negative forms of condition that are more English-like.


File: exim_filter.info,  Node: String testing conditions,  Next: Numeric testing conditions,  Prev: Obeying commands conditionally,  Up: Exim's interface to mail filtering

String testing conditions
=========================

There are a number of conditions that operate on text strings, using
the words `begins', `ends', `is', `contains' and `matches'. If the
condition names are written in lower case, the testing of letters is
done without regard to case; if they are written in upper case (for
example, `CONTAINS') then the case of letters is significant.

          <text1> begins <text2>
          <text1> does not begin <text2>
     e.g. $header_from: begins "Friend@"

A `begins' test checks for the presence of the second string at the
start of the first, both strings having been expanded.

          <text1> ends <text2>
          <text1> does not end <text2>
     e.g. $header_from: ends "public.com.example"

An `ends' test checks for the presence of the second string at the end
of the first, both strings having been expanded.

          <text1> is <text2>
          <text1> is not <text2>
     e.g. $local_part_suffix is "-foo"

An `is' test does an exact match between the strings, having first
expanded both strings.

          <text1> contains <text2>
          <text1> does not contain <text2>
     e.g. $header_subject: contains "evolution"

A `contains' test does a partial string match, having expanded both
strings.

          <text1> matches <text2>
          <text2> does not match <text2>
     e.g. $sender_address matches "(Bill|John)@"

For a `matches' test, after expansion of both strings, the second one is
interpreted as a regular expression. Exim uses the PCRE regular
expression library, which provides regular expressions that are
compatible with Perl.

Care must be taken if you need a backslash in a regular expression,
because backslashes are interpreted as escape characters both by the
string expansion code and by Exim's normal processing of strings in
quotes. For example, if you want to test the sender address for a
domain ending in `.com' the regular expression is

     \.com$

The backslash and dollar sign in that expression have to be escaped
when used in a filter command, as otherwise they would be interpreted
by the expansion code. Thus what you actually write is

     if $sender_address matches \\.com\$

An alternative way of handling this is to make use of the `\N' expansion
flag for suppressing expansion:

     if $sender_address matches \N\.com$\N

Everything between the two occurrences of `\N' is copied without change
by the string expander (and in fact you do not need the final one,
because it is at the end of the string).

If the regular expression is given in quotes (mandatory only if it
contains white space) you have to write either

     if $sender_address matches "\\\\.com\\$"

or

     if $sender_address matches "\\N\\.com$\\N"

If the regular expression contains bracketed sub-expressions, numeric
variable substitutions such as $1 can be used in the subsequent actions
after a successful match. If the match fails, the values of the numeric
variables remain unchanged. Previous values are not restored after
`endif' - in other words, only one set of values is ever available. If
the condition contains several sub-conditions connected by `and' or
`or', it is the strings extracted from the last successful match that
are available in subsequent actions. Numeric variables from any one
sub-condition are also available for use in subsequent sub-conditions,
since string expansion of a condition occurs just before it is tested.


File: exim_filter.info,  Node: Numeric testing conditions,  Next: Testing for personal mail,  Prev: String testing conditions,  Up: Exim's interface to mail filtering

Numeric testing conditions
==========================

The following conditions are available for performing numerical tests:

          <number1> is above <number2>
          <number1> is not above <number2>
          <number1> is below <number2>
          <number1> is not below <number2>
     e.g. $message_size is not above 10k

The <number> arguments must expand to strings of digits, optionally
followed by one of the letters K or M (upper case or lower case) which
cause multiplication by 1024 and 1024x1024 respectively.


File: exim_filter.info,  Node: Testing for personal mail,  Next: Testing for significant deliveries,  Prev: Numeric testing conditions,  Up: Exim's interface to mail filtering

Testing for personal mail
=========================

A common requirement is to distinguish between incoming personal mail
and mail from a mailing list. In particular, this test is normally
required for so-called `vacation messages'. The condition

          personal

is a shorthand for

          $header_to: contains "$local_part@$domain" and
          $header_from: does not contain "$local_part@$domain" and
          $header_from: does not contain "server@" and
          $header_from: does not contain "daemon@" and
          $header_from: does not contain "root@" and
          $header_subject: does not contain "circular" and
          $header_precedence: does not contain "bulk" and
          $header_precedence: does not contain "list" and
          $header_precedence: does not contain "junk"

The variable $local_part contains the local part of the mail address of
the user whose filter file is being run - it is normally your login id.
The $domain variable contains the mail domain. This condition tests for
the appearance of the current user in the `To:' header, checks that the
sender is not the current user or one of a number of common daemons,
and checks the content of the `Subject:' and `Precedence:' headers.

If prefixes or suffixes are in use for local parts - something which
depends on the configuration of Exim (see section "Multiple personal
mailboxes" below) - the first two tests above are also done with

     $local_part_prefix$local_part$local_part_suffix

instead of just $local_part. If the system is configured to rewrite
local parts of mail addresses, for example, to rewrite `dag46' as
`Dirk.Gently', the rewritten form of the address is also used in the
tests.

This example shows the use of `personal' in a filter file that is
sending out vacation messages:

     if personal then
       mail
        to $reply_address
        subject "Re: $h_subject:"
        file $home/vacation/message
        once $home/vacation/once
        once_repeat 10d
     endif

It is quite common for people who have mail accounts on a number of
different systems to forward all their mail to one system, and in this
case a check for personal mail should test all their various mail
addresses. To allow for this, the `personal' condition keyword can be
followed by

     alias <address>

any number of times, for example

     if personal alias smith@else.where.example
                 alias jones@other.place.example
     then ...

This causes messages containing the alias addresses in any places where
the local address is tested to be treated as personal.


File: exim_filter.info,  Node: Testing for significant deliveries,  Next: Testing for error messages,  Prev: Testing for personal mail,  Up: Exim's interface to mail filtering

Testing for significant deliveries
==================================

Whether or not any previously obeyed filter commands have resulted in a
significant delivery can be tested by the condition `delivered', for
example:

     if not delivered then save mail/anomalous endif


File: exim_filter.info,  Node: Testing for error messages,  Next: Testing delivery status,  Prev: Testing for significant deliveries,  Up: Exim's interface to mail filtering

Testing for error messages
==========================

The condition `error_message' is true if the incoming message is a mail
delivery error message (bounce message). Putting the command

     if error_message then finish endif

at the head of your filter file is a useful insurance against things
going wrong in such a way that you cannot receive delivery error
reports, and is highly recommended. Note that `error_message' is a
condition, not an expansion variable, and therefore is not preceded by
"$".


File: exim_filter.info,  Node: Testing delivery status,  Next: Testing a list of addresses,  Prev: Testing for error messages,  Up: Exim's interface to mail filtering

Testing delivery status
=======================

There are two conditions which are intended mainly for use in system
filter files, but which are available in users' filter files as well.
The condition `first_delivery' is true if this is the first attempt to
deliver the message, and false otherwise. In a user filter file it will
be false only if there was previously an error in the filter, or if a
delivery for the user failed owing to, for example, a quota error, or
forwarding to a remote address that was deferred for some reason.

The condition `manually_thawed' is true only if the message was
`frozen' for some reason, and was subsequently released by the system
administrator. It is unlikely to be of use in users' filter files.


File: exim_filter.info,  Node: Testing a list of addresses,  Next: Multiple personal mailboxes,  Prev: Testing delivery status,  Up: Exim's interface to mail filtering

Testing a list of addresses
===========================

There is a facility for looping through a list of addresses and
applying a condition to each of them. It takes the form

     foranyaddress <string> (<condition>)

where <string> is interpreted as a list of RFC 2822 addresses, as in a
typical header line, and <condition> is any valid filter condition or
combination of conditions. The `group' syntax that is defined for
certain header lines that contain addresses is supported.

The parentheses surrounding the condition are mandatory, to delimit it
from possible further sub-conditions of the enclosing `if' command.
Within the condition, the expansion variable $thisaddress is set to the
non-comment portion of each of the addresses in the string in turn. For
example, if the string is

     B.Simpson <bart@sfld.example>, lisa@sfld.example (his sister)

then $thisaddress would take on the values `bart@sfld.example' and
`lisa@sfld.example' in turn.

If there are no valid addresses in the list, the whole condition is
false. If the internal condition is true for any one address, the
overall condition is true and the loop ends. If the internal condition
is false for all addresses in the list, the overall condition is false.
This example tests for the presence of an eight-digit local part in any
address in a "To:" header:

     if foranyaddress $h_to: ( $thisaddress matches ^\\d{8}@ ) then ...

When the overall condition is true, the value of $thisaddress in the
commands that follow `then' is the last value it took on inside the
loop. At the end of the `if' command, the value of $thisaddress is
reset to what it was before. It is best to avoid the use of multiple
occurrences of `foranyaddress', nested or otherwise, in a single `if'
command, if the value of $thisaddress is to be used afterwards, because
it isn't always clear what the value will be. Nested `if' commands
should be used instead.

Header lines can be joined together if a check is to be applied to more
than one of them. For example:

     if foranyaddress $h_to:,$h_cc: ....

scans through the addresses in both the "To:" and the "Cc:" headers.


File: exim_filter.info,  Node: Multiple personal mailboxes,  Next: Ignoring delivery errors,  Prev: Testing a list of addresses,  Up: Exim's interface to mail filtering

Multiple personal mailboxes
===========================

The system administrator can configure Exim so that users can set up
variants on their email addresses and handle them separately. Consult
your system administrator or local documentation to see if this
facility is enabled on your system, and if so, what the details are.

The facility involves the use of a prefix or a suffix on an email
address. For example, all mail addressed to "lg103-<something>" would
be the property of user "lg103", who could determine how it was to be
handled, depending on the value of <something>.

There are two possible ways in which this can be set up. The first
possibility is the use of multiple `.forward' files. In this case, mail
to "lg103-foo", for example, is handled by looking for a file called
".forward-foo" in "lg103's" home directory. If such a file does not
exist, delivery fails and the message is returned to its sender.

The alternative approach is to pass all messages through a single
`.forward' file, which must be a filter file in order to distinguish
between the different cases by referencing the variables
$local_part_prefix or $local_part_suffix, as in the final example in
section "Examples of filter commands" below. If the filter file does
not handle a prefixed or suffixed address, delivery fails and the
message is returned to its sender.

It is possible to configure Exim to support both schemes at once. In
this case, a specific ".forward-foo" file is first sought; if it is not
found, the basic `.forward' file is used.

The `personal' test (see section "Testing for personal mail") includes
prefixes and suffixes in its checking.


File: exim_filter.info,  Node: Ignoring delivery errors,  Next: Examples of filter commands,  Prev: Multiple personal mailboxes,  Up: Exim's interface to mail filtering

Ignoring delivery errors
========================

As was explained above, filtering just sets up addresses for delivery -
no deliveries are actually done while a filter file is active. If any
of the generated addresses subsequently suffers a delivery failure, an
error message is generated in the normal way. However, if the filter
command which sets up a delivery is preceded by the word `noerror',
errors for that delivery, and any deliveries consequent on it (that is,
from alias, forwarding, or filter files it invokes) are ignored.


File: exim_filter.info,  Node: Examples of filter commands,  Prev: Ignoring delivery errors,  Up: Exim's interface to mail filtering

Examples of filter commands
===========================

Simple forwarding:

     # Exim filter
     deliver baggins@rivendell.middle-earth.example

Vacation handling using traditional means, assuming that the
".vacation.msg" and other files have been set up in your home directory:

     # Exim filter
     unseen pipe "/usr/ucb/vacation \"$local_part\""

Vacation handling inside Exim, having first created a file called
".vacation.msg" in your home directory:

     # Exim filter
     if personal then vacation endif

File some messages by subject:

     # Exim filter
     if $header_subject: contains "empire" or
        $header_subject: contains "foundation"
     then
        save $home/mail/f+e
     endif

Save all non-urgent messages by weekday:

     # Exim filter
     if $header_subject: does not contain "urgent" and
        $tod_full matches "^(...),"
     then
       save $home/mail/$1
     endif

Throw away all mail from one site, except from postmaster:

     # Exim filter
     if $reply_address contains "@spam.site.example" and
        $reply_address does not contain "postmaster@"
     then
        seen finish
     endif

Handle multiple personal mailboxes

     # Exim filter
     if $local_part_suffix is "-foo"
     then
       save $home/mail/foo
     elif $local_part_suffix is "-bar"
     then
       save $home/mail/bar
     endif



Tag Table:
Node: Top88
Node: Exim's interface to mail filtering974
Node: Introduction2295
Node: Testing a new filter file4026
Node: Installing a filter file8366
Node: Testing an installed filter file8912
Node: Format of filter files10404
Node: String expansion13576
Node: Some useful general variables15104
Node: Header variables18624
Node: User variables20444
Node: Significant deliveries21107
Node: Filter commands22356
Node: The add command23984
Node: The deliver command24599
Node: The save command26734
Node: The pipe command28948
Node: Mail commands35200
Node: Logging commands40707
Node: The finish command42796
Node: The testprint command43202
Node: The fail command43826
Node: The freeze command44348
Node: Obeying commands conditionally44892
Node: String testing conditions46257
Node: Numeric testing conditions49868
Node: Testing for personal mail50568
Node: Testing for significant deliveries53329
Node: Testing for error messages53784
Node: Testing delivery status54469
Node: Testing a list of addresses55379
Node: Multiple personal mailboxes57685
Node: Ignoring delivery errors59511
Node: Examples of filter commands60222

End Tag Table
