

                     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.
___________________________________________________________________________



CONTENTS

  1. Introduction
  2. Testing a new filter file
  3. Installing a filter file
  4. Testing an installed filter file
  5. Format of filter files
  6. String expansion
  7. Some useful general variables
  8. Header variables
  9. User variables
  10. Significant deliveries
  11. Filter commands
  12. The add command
  13. The deliver command
  14. The save command
  15. The pipe command
  16. Mail commands
  17. Logging commands
  18. The finish command
  19. The testprint command
  20. The fail command
  21. The freeze command
  22. Obeying commands conditionally
  23. String testing conditions
  24. Numeric testing conditions
  25. Testing for personal mail
  26. Testing for significant deliveries
  27. Testing for error messages
  28. Testing delivery status
  29. Testing a list of addresses
  30. Multiple personal mailboxes
  31. Ignoring delivery errors
  32. Examples of filter commands


1. Introduction

Most Unix mail transfer agents (programs that deliver mail) permit individ-
ual 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 instruc-
tions 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 gener-
ated 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.


2. 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 30 below.


3. 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.


4. 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.


5. 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.


6. String expansion

Most data values are expanded before use. Expansion consists of replacing
substrings beginning with "$" with other text. The full expansion facili-
ties 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.


7. Some useful general variables

A complete list of the available variables is given in the Exim documen-
tation. 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 30 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 30 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 30 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.


8. 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".


9. User variables

There are ten user variables with names $n0 - $n9 that can be incremented
by the "add" command (see section 12). 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.


10. 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 signifi-
cant 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 signifi-
cant. 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".


11. 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 12)
  "deliver"        deliver to an email address (section 13)
  "fail"           force delivery failure (sysadmin use) (section 20)
  "finish"         end processing (section 18)
  "freeze"         freeze message (sysadmin use) (section 21)
  "if"             test condition(s) (section 22)
  "logfile"        define log file (section 17)
  "logwrite"       write to log file (section 17)
  "mail"           send a reply message (section 16)
  "pipe"           pipe to a command (section 15)
  "save"           save to a file (section 14)
  "testprint"      print while testing (section 19)
  "vacation"       tailored form of "mail" (section 16)

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.


12. 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.


13. 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 pro-
cessed 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.


14. 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.


15. 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 argu-
ments, 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 deliver-
ies 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.


16. 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 25 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 6 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.


17. 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.


18. 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.


19. 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 2 above), the value
of the string is written to the standard output.


20. 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.


21. 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.


22. 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.


23. 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.


24. 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.


25. 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 30 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.


26. 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


27. 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 "$".


28. 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.


29. 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.


30. 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 ".for-
ward-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 32 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 25) includes prefixes and suffixes in its
checking.


31. 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.


32. 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

