

                   EXIM: USER INTERFACE TO MAIL FILTERING


Exim is a mail transfer agent for Unix systems. This document describes the
user interface to its inbuilt mail filtering facility, and is copyright (c)
University of Cambridge 1996.
___________________________________________________________________________


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 at all has to be enabled by the system
administrator; some of the individual facilities can be separately
disabled.

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, and this should
be used first. Because a filter can do tests on the content of messages, a
test message is required. Suppose you have a new filter file called "new-
filter" and a test message called "test-message". Assuming that Exim is
installed with the conventional path name /usr/lib/sendmail, the following
command can be used:

  /usr/lib/sendmail -bf test-filter <test-message

The -bf option tells Exim that the following file is a filter to be tested,
and the test message is supplied on the standard input. If there are no
message-dependent tests in the filter, then an empty file can be supplied.
The result of this command, provided no errors are detected, 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.book
  Save message to: /home/lemuel/mail/archive

means that one copy of the message would be sent to
gulliver@lilliput.fict.book, and another would be added to the file
/home/lemuel/mail/archive.

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 if
nesting of if commands. Further additional output from a filter test can be
generated by the testprint command, which is described below.

When testing a filter in this way, Exim makes up an 'envelope' for the
message. The recipient is always the user running the command, and so is
the sender, unless the message contains a "Return-path:" header, or the
command is run with the -f option to supply an 'envelope sender'. You need
worry about this only if you want to test out features of your filter file
that rely on the return path.


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, then 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.


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. The white
space 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, separated by white space and or line
breaks, except in the case of conditions for the "if" command, where round
brackets (parentheses) also act as separators. For example, in the command

  deliver gulliver@lilliput.fict.book

the keyword is "deliver" and the data value is
"gulliver@lilliput.fict.book". The commands are in free format, and there
are no special terminators. If the character # follows a separator, then
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 data values 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
     hexacimal digits is treated similarly. Backslash followed by any other
     character is replaced by the second character, so that in particular,
     \" becomes " and \\ becomes \.

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 character $ is also
significant.


6. String expansion

Most data values are expanded before use. Expansion consists of replacing
substrings beginning with $ with other text. The full expansion facilities
are described in section 15 below, but the most common case is the
substitution of a simple 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}

The variables most likely to be useful in filter files are:

home: The user's home directory.

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

message_size: The size of the message, in bytes.

reply_address: The address from the "Reply-to:" header, if the message has
one; otherwise the address from 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 is sent as
part of the message's envelope, and which is the address to which delivery
errors are sent. In many cases, this has the same value as sender_address,
but if, for example, an incoming message to a mailing list has been
expanded, then return_path may contain the address of the list maintainer
instead.

sender_address: The sender address that was received with the message.

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.

In addition to these 'ordinary' variables, there is a special set of
variables containing the headers of the message being processed. These
variables have names beginning with "$header_" followed by the name of the
header, terminated by a colon. The whole item, including the terminating
colon, is replaced by the contents of the message header. If there is more
than one header with the same name, their contents are concatenated, with a
single newline character between them. For example,

  $header_from:
  $header_subject:

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 822, 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.

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".


7. Significant deliveries

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

The delivery commands described in the next section are by default
significant. However, if such a command is preceded by the word "unseen",
then 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".


8. Delivery commands

There are three commands that cause a copy of the message to be
transmitted:

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

This is a mail forwarding operation. The message is sent on to the given
address, exactly as happens if the address had appeared in a traditional
.forward file.

       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, mail folder). If the name does not start with a / character, then the
contents of the $home variable are prepended. The user must of course have
permission to write to the file. In addition, the ability to use this
command is controlled by the system administrator - it may be forbidden on
some systems.

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

This command causes a separate process to be run, and a copy of the message  |
is passed on its standard input. The command supplied to pipe is split up    |
by Exim into a command name and a number of arguments, delimited by white    |
space except for arguments enclosed in double quotes, in which case          |
backslash is interpreted as an escape. Note that as the whole command is     |
normally supplied in double quotes, a second level of quoting is required.   |
For example:                                                                 |
                                                                             |
       pipe "$home/myscript \"size is $message_size\""                       |
                                                                             |
String expansion is performed on the separate components after the line has  |
been split up. Therefore substitution cannot change the number of argu-      |
ments, nor can quotes and backslashes in variables cause confusion. The      |
command is run directly by Exim; it is not run under a shell.                |
                                                                             |
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, the following environment variables are set up:     |
                                                                             |
  DOMAIN               the local domain of the address                       |
  HOME                 your home directory                                   |
  LOCAL_PART           your login name                                       |
  LOGNAME              your login name                                       |
  MESSAGE_ID           the message's unique id                               |
  PATH                 the command search path                               |
  SENDER               the sender of the message                             |
  SHELL                /bin/sh                                               |
  USER                 your login name                                       |
                                                                             |
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 822 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. Always use quotes, for example:                                         |
                                                                             |
  /some/command '$SENDER'                                                    |
                                                                             |
so that inserted shell meta-characters do not cause unwanted effects.        |


9. Mail commands

There are two commands which cause the creation of a new mail message,
which does not 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>]
            [subject <text>]
            [text <text>]
            [[expand] file <filename>]
            [log <log file name>]
            [once <note file name>]

  e.g. mail text "Your message re $h_subject 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 if no file name
is specified, an expanded ".vacation.msg" is used, if no log is given,
".vacation.log" is used, and if no "once" file is given ".vacation" is
used. This mimics the behaviour of the traditional Unix vacation command.
If a file name is given to "vacation", it is expanded only if explicitly
requested.

An "In-Reply-To:" header is automatically included in the created message,
giving a reference to the message identification of the incoming message.
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",
then each line of the file is subject to string expansion as it is included
in the message.

If no "to" argument appears, the message is sent to the address in the
"Reply_address" variable (see section 6 above). 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 create a database for remembering who has received a message,
and no more than one message is ever sent to any particular address.


10. Other commands

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.

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, the value of the string is
written to the standard output.


11. 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"-"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". A condition can
be preceded by "not" to negate it, and there are also some negative forms
of condition that are more English-like. The following conditions are
available:

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

This does a simple string match without regard to the case of letters,
having expanded both strings.

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

After expansion of both strings, the second one is interpreted as a regular
expression, but the matching is done independent of case. The syntax of
regular expressions supported by Exim is described in section 14 below.
Note that if you need a backslash in the expression and it is quoted, you
must use \\ because the string is subject to normal Exim escape processing.
Note also that since the regular expression string is expanded, if you need
a $ in the expression, it must be escaped, and because of the previous
comment, you need to use \\$, not just \$ if the string is in quotes.

If the regular expression contains bracketed subexpressions, then numeric
variables such as $1 can be used in the subsequent actions, up to the next
"endif". Previous values are not restored after "endif" - in other words,
only one set of values is ever available. If the condition contains several
subconditions connected by "and" or "or", it is the subexpressions from the
last regular expression match that are available.

       <number1> above <number1>
       <number1> is not above <number1>
       <number1> below <number1>
       <number1> is not below <number1>
  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 (in either case) which cause
multiplication by 1024 and 1024x1024 respectively.

A common requirement is to distinguish between incoming personal mail and
mail from a mailing list. 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"

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 the system is configured to rewrite local parts of mail addresses, for
example, to rewrite 'dag46' as 'Dirk.Gently', then the rewritten form of
the address is also used in the tests.

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

  personal alias smith@else.where alias jones@other.place

Whether or not any previously obeyed filter commands have resulted in
significant actions can be tested by the condition "delivered", for
example:

  if not delivered then save mail/anomalous endif

Finally, the condition "error_message" is true if the incoming message is a
mail delivery error 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.


12. 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", then errors for
that delivery, and any deliveries consequent on it (that is, from alias,
forwarding, or filter files it invokes) are ignored.


13. Examples of filter commands

Simple forwarding:

  # Exim filter
  deliver baggins@rivendell.middle.earth

Vacation handling using traditional means:

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

Vacation handling inside Exim:

  # 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" and
     $reply_address does not contain "postmaster@"
  then
     seen finish
  endif


14. Regular expressions

Exim uses Henry Spencer's freely distributable regular expression library.
The syntax of the regular expressions that it supports is as follows:

A regular expression is zero or more branches, separated by '|'. It matches
anything that matches one of the branches. A branch is zero or more pieces,
concatenated. It matches a match for the first, followed by a match for the
second, etc. A piece is an atom possibly followed by '*', '+', or '?'.

An atom followed by '*' matches a sequence of 0 or more matches of the
atom. An atom followed by '+' matches a sequence of 1 or more matches of
the atom. An atom followed by '?' matches a match of the atom, or the null
string.

An atom is a regular expression in parentheses (matching a match for the
regular expression), a range (see below), '.' (matching any single charac-
ter), '^' (matching the null string at the beginning of the input string),
'$' (matching the null string at the end of the input string), a '\'
followed by a single character (matching that character), or a single
character with no other significance (matching that character).

A range is a sequence of characters enclosed in '[]'. It normally matches
any single character from the sequence. If the sequence begins with '^', it
matches any single character not from the rest of the sequence. If two
characters in the sequence are separated by '-', this is shorthand for the
full list of ASCII characters between them (e.g. '[0-9]' matches any
decimal digit). To include a literal ']' in the sequence, make it the first
character (following a possible '^'). To include a literal '-', make it the
first or last character.


15. More about string expansion

The description which follows is an excerpt from the full specification of
Exim. It refers to a number of expansion variables that are mainly of
interest to the system administrator. Not all of them are relevant when
processing filter files.

Expanded strings are copied verbatim except when a dollar character is
encountered. This specifies the start of a portion of the string which is
interpreted and replaced as described below.

An uninterpreted dollar can be included in the string by putting a
backslash in front of it - if the string appears in quotes, two backslashes
are required because the quotes themselves cause some interpretation when
the string is read in. A backslash can in fact be used to prevent any
character being treated specially in an expansion.


16. Expansion items

The following items are recognized in expanded strings:

$<variable name> or ${<variable name>}

   Substitute the contents of the named variable; the latter form can be
   used to separate the name from subsequent alphameric characters. The
   names of the variables are given in section 19 below.

$header_<header name>: or $h_<header name>:

   Substitute the contents of the named message header, for example

     $header_reply-to:

   This particular expansion is intended mainly for use in user's filter
   files. The header names follow the syntax of RFC 822, which states that
   they may contain any printing characters except space and colon.
   Consequently, curly brackets do not terminate header names. Upper and
   lower case letters are synonymous in header names. If the following
   character is white space, the terminating colon may be omitted. If the
   message does not contain the given header, the expansion item is
   replaced by an empty string. If there is more than one header with the
   same name, they are all concatenated to form the substitution string,
   with a newline character between them.

${<op>:<string>}

   The string is first itself expanded, and then the operation specified by
   <op> is applied to it. A list of operators is given in section 17 below.

${if <condition> {<string1>}{<string2>}}

   If <condition> is true, <string1> is expanded and replaces the whole
   item; otherwise <string2> is used. The second string need not be
   present; if it is not and the condition is not true, the item is
   replaced with nothing. Alternatively, the word 'fail' may be present
   instead of the second string (without any curly brackets). In this case,
   the expansion fails if the condition is not true. The available
   conditions are described in section 18 below.

${lookup{<key>} <search type> {<file>} {<string1>} {<string2>}}

   The <key> and <file> strings are first expanded. The key is then looked
   up in the file, using the given search type, which can be one of

     dbm       do a DBM lookup
     lsearch   do a linear search
     nis       search a NIS map

   For a linear search, a line beginning with the key followed by a colon
   is searched for, and the data is the remainder of the line and any
   continuations, in the format of an alias file. For a NIS search, <file>
   is the name of the NIS map. The NIS facility is new in Exim 0.43 and has
   not been used extensively.

   If the key is found, then <string1> is expanded and replaces the entire
   item. During its expansion, a variable called value is available,
   containing the data returned by the file lookup. If the key is not
   found, then <string2> is expanded and replaces the entire item. It may
   be omitted, in which case the replacement is null.

   Instead of {<string2>} the word 'fail' can appear, and in this case, if
   the key is not found in the file, the string expansion fails in a way
   that can be detected by the caller. The consequences of this depend on
   the circumstances.

${lookup{<key:subkey>} <search type> {<file>} {<string1>} {<string2>}}

   This searches for <key> in the file as described above; if it succeeds,
   it extracts from the data a subfield which is identified by the
   <subkey>. The data related to the main key must be of the form:

     <subkey1> = <value1>  <subkey2> = <value2> ...

   where the equals signs are optional. If any of the values contain white
   space, they must be enclosed in double quotes, and any values that are
   enclosed in double quotes are subject to escape processing as described
   in section 5. For example, if a line in a linearly searched file
   contains

     alice: uid=1984 gid=2001

   then expanding the string

     ${lookup{alice:uid}lsearch{<file name>}{$value}}

   yields the string '1984'. If the subkey is not found in <string1>, then
   <string2> is expanded and replaces the entire item.

${extract{<key>}{<string>}}

   The key and the string are first expanded. Then the subfield identified
   by the key is extracted from the string, exactly as just described for
   lookup items with subkeys. If the key is not found in the string, the
   item is replaced by nothing.


17. Expansion operators

A string can be forced into lower case by the lc operator, for example

  ${lc:$local_part}

The length operator can be used to extract the initial portion of a string.
It is followed by an underscore and the number of characters required. For
example

  ${length_50:$message_body}

The result of this operator is either the first n characters or the whole
string, whichever is the shorter. The abbreviation l can be used instead of
length.


18. Expansion conditions

The following conditions are available for testing while expanding strings:

  !<condition>

This negates the result of the condition.

  def:<variable>

This condition is true if the named expansion variable does not contain the
empty string.

  exists:<file name>

This condition is true if the named file (or directory) exists. The
existence test is done by calling the stat() function.

  eq {string1}{string2}

This condition is true if the two strings are identical, including the case
of letters.

  or {{cond1}{cond2}...}

This condition is true if any one of the sub-conditions is true.

  and {{cond1}{cond2}...}

This condition is true if all of the sub-conditions are true.


19. Expansion variables

The variables that are available for use in expansion strings are:

caller_gid: The group id under which the process that called Exim was
running.

caller_uid: The user id under which the process that called Exim was
running.

compile_date: The date on which the Exim binary was compiled.

compile_number: The building process for Exim keeps a count of the number
of times it has been compiled. This serves to distinguish different
compilations of the same version of the program.

domain: When an address is being directed, routed, or a local delivery is
taking place, this variable contains the domain. When a remote delivery is
taking place, if all the addresses that are being handled simultaneously
contain the same domain, then it is placed in the domain variable.
Otherwise this variable is empty. Remote transports should be restricted to
handling only one domain at once if its value is required at transport
time.

home: The forwardfile director sets this variable to the value of the
directory which contains the file (normally the user's home directory)
while processing a filter file. A home directory may also be set during a
local delivery, either by the transport or by the director that handled the
address. When this is the case, the home variable contains its value. When
running a filter test via the -bf option, home is set to the value of the
environment variable HOME.

host: When a local transport is run as a result of routing a remote
address, the expansion variable host is available to access the host that
the router defined. A DNS lookup may set up many hosts; in this case host
refers to the first one. It is expected that this usage will be mainly via
the domainlist router, setting up a single host for batched SMTP output.

local_part: When an address is being directed, routed, or delivered
locally, this variable contains the local part.

message_body: This variable contains the initial portion of a message's
body while it is being delivered, and is intended mainly for use in filter
files. The maximum number of characters of the body that are used is set by
the message_body_visible configuration option; the default is 500.

message_id: When a message is being received or delivered, this variable
contains the message's id.

message_size: When a message is being received or delivered, this variable
contains its size in bytes.

primary_hostname: The value set in the configuration file, or read by the
uname() function.

received_protocol: When a message is being processed, this variable con-
tains the protocol by which it was received.

reply_address: When a message is being processed, this variable contains
the contents of the Reply-to: header if one exists, or otherwise the
contents of the From: header.

return_path: When a message is being delivered, this variable contains the
return path - the sender field that is sent as part of the envelope. In
many cases, this has the same value as sender_address, but if, for example,
an incoming message to a mailing list has been expanded by a director which
specifies a specific address for delivery error messages, then return_path
contains the new errors address, while sender_address contains the original
sender address that was received with the message.

sender_address: When a message is being processed, this variable contains
the sender's address that was received in the message's envelope.

sender_fullhost: When a message has been received from a remote host, this
variable contains the host name and IP address, as a concatenated string,
with the IP address in square brackets. In the case of incoming SMTP
messages, the host name is the data receceived in the HELO or EHLO command.

sender_host_address: When a message has been received from a remote host,
this variable contains the host's IP address.

sender_host_name: When a message has been received from a remote host, this
variable contains the host's name (from the HELO or EHLO command, in the
case of SMTP).

sender_ident: When a message has been received from a remote host, this
variable contains the identification received in response to an RFC 1413
request. When a message has been received locally, this variable contains
the login name of the user that called Exim.

spool_directory: The name of Exim's spool directory.

tod_bsdinbox: The time of day and date, in the format required for BSD-
style mailbox files, for example: Thu Oct 17 17:14:09 1995.

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,
which is: 1995-10-12 15:32:29.

value: This variable contains the result of an expansion lookup operation,
as described above. If used in other circumstances, its contents are null.

version_number: The version number of Exim.
