Exim Filter Specification chapter 3

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# 3. Exim filter files

This chapter contains a full description of the contents of Exim filter files.

## 3.1. Format of Exim 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 or a Sieve filter file. If the file does not have this initial line (or the equivalent for a Sieve filter), 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.

## 3.2. Data values in filter commands

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.

The maximum permitted length of a data string, before expansion, is 1024 characters.

## 3.3. String expansion

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

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

      $reply_address

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

      ${reply_address}

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

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

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

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

tests for a run of four dollar characters.

## 3.4. Some useful general variables

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

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

`$home`: In conventional configurations, this variable normally contains the user's home directory. The system administrator can, however, change this.

`$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 3.29 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 3.29 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 3.29 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, without the timezone, for example: 1995-10-12 15:32:29.

`$tod_zone`: The local timezone offset, for example: +0100.

## 3.5. 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 line, 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.

Leading and trailing white space is removed from header line data, and if there are any MIME “words” that are encoded as defined by RFC 2047 (because they contain non-ASCII characters), they are decoded and translated, if possible, to a local character set. Translation is attempted only on operating systems that have the `iconv()` function. This makes the header line look the same as it would when displayed by an MUA. The default character set is ISO-8859-1, but this can be changed by means of the `headers` command (see below).

If you want to see the actual characters that make up a header line, you can specify `$rheader_` instead of `$header_`. This inserts the “raw” header line, unmodified.

There is also an intermediate form, requested by `$bheader_`, which removes leading and trailing space and decodes MIME “words”, but does not do any character translation. If an attempt to decode what looks superficially like a MIME “word” fails, the raw string is returned. If decoding produces a binary zero character, it is replaced by a question mark.

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 `$header_`, `$rheader_`, or `$bheader_` can be abbreviated to `$h_`, `$rh_`, or `$bh_`, respectively.

- 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`.

## 3.6. User variables

There are ten user variables with names `$n0` – `$n9` that can be incremented by the `add` command (see section 3.10). 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.

## 3.7. Current directory

The contents of your filter file should not make any assumptions about the current directory. It is best to use absolute paths for file names; you can normally make use of the `$home` variable to refer to your home directory. The `save` command automatically inserts `$home` at the start of non-absolute paths.

## 3.8. Significant deliveries

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

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

## 3.9. 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 3.10)
`deliver` | deliver to an email address (section 3.11)
`fail` | force delivery failure (sysadmin use) (section 3.18)
`finish` | end processing (section 3.16)
`freeze` | freeze message (sysadmin use) (section 3.19)
`headers` | set the header character set (section 3.20)
`if` | test condition(s) (section 3.21)
`logfile` | define log file (section 3.15)
`logwrite` | write to log file (section 3.15)
`mail` | send a reply message (section 3.14)
`pipe` | pipe to a command (section 3.13)
`save` | save to a file (section 3.12)
`testprint` | print while testing (section 3.17)
`vacation` | tailored form of `mail` (section 3.14)

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.

## 3.10. 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.

## 3.11. The deliver command

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

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

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

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

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

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

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

## 3.12. 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 `$home` variable are prepended, unless it is empty. In conventional configurations, this variable is normally set in a user filter to the user's home directory, but the system administrator may set it to some other path. In some configurations, `$home` may be unset, in which case a non-absolute path name may be generated. Such configurations convert this to an absolute path when the delivery takes place. In a system filter, `$home` is never set.

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.

## 3.13. 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).

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

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

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

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

      IFS=" "

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

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

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

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

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

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

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

      /some/command '$SENDER'

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

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

A consequence of this is that you cannot inspect the return code from the pipe command from within the filter. Nevertheless, 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 disk, 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.

## 3.14. 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>]`
`` `[extra_headers <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"`

Each <address-list> can contain a number of addresses, separated by commas, in the format of a To: or Cc: header line. In fact, the text you supply here is copied exactly into the appropriate header line. Thus, it may contain additional information as well as email addresses. For example:

      mail to "Julius Caesar <jc@rome.example>, \
               <ma@rome.example> (Mark A.)"

Similarly, the texts supplied for From: and Reply-to: are copied into their respective header lines.

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 `subject`, `file`, `log`, `once`, and `once_repeat` options are

`` `subject "On vacation"`
`` `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.

Warning: The `vacation` command should always be used conditionally, subject to at least the `personal` condition (see section 3.26 below) so as not to send automatic replies to non-personal messages from mailing lists or elsewhere. Sending an automatic response to a mailing list or a mailing list manager is an Internet Sin.

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.

The `extra_headers` keyword allows you to add custom header lines to the message. The text supplied must be one or more syntactically valid RFC 2882 header lines. You can use “\n” within quoted text to specify newlines between headers, and also to define continued header lines. For example:

      extra_headers "h1: first\nh2: second\n continued\nh3: third"

No newline should appear at the end of the final header line.

If no `to` argument appears, the message is sent to the address in the `$reply_address` variable (see section 3.3 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.

## 3.15. 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.

## 3.16. 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.

## 3.17. 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 1.2 above), the value of the string is written to the standard output.

## 3.18. 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.

## 3.19. 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.

## 3.20. The headers command

The `headers` command can be used to change the target character set which is used when translating the contents of encoded header lines for insertion by the `$header_` mechanism (see section 3.5 above). The default can be set in the Exim configuration; if not specified, ISO-8859-1 is used. The only currently supported format for the `headers` command is as in this example:

      headers charset "UTF-8"

That is, `headers` is followed by the word `charset` and then the name of a character set. This particular example would be useful if you wanted to compare the contents of a header to a UTF-8 string.

## 3.21. 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.

## 3.22. String testing conditions

There are a number of conditions that operate on text strings, using the words “begins”, “ends”, “is”, “contains” and “matches”.

Note that if you want to apply the same test to more than one header line, you can easily concatenate them into a single string for testing, as in this example:

      if "$h_to:, $h_cc:" contains me@domain.example then ...

If any of 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>`
`` `<text1> 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.

The match succeeds if the regular expression matches any part of the first string. If you want a regular expression to match only at the start or end of the subject string, you must encode that requirement explicitly, using the ^ or $ metacharacters. The above example, which is not so constrained, matches all these addresses:

      bill@test.example
      john@some.example
      spoonbill@example.com
      littlejohn@example.com

To match only the first two, you could use this:

      if $sender_address matches "^(bill|john)@" then ...

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.

## 3.23. 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.

## 3.24. 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

## 3.25. 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 `$`.

## 3.26. Testing for personal mail

A common requirement is to distinguish between incoming personal mail and mail from a mailing list, or from a robot or other automatic process (for example, a bounce message). In particular, this test is normally required for so-called “vacation messages”. The condition

`` `personal`

is a shorthand for

`` `not error_message and`
`` `$header_to: contains "$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_auto-submitted: does not contain "auto-" 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 3.29 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.

## 3.27. 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. This indicator is not reset until the first delivery process successfully terminates; if there is a crash or a power failure (for example), the next delivery attempt is also a “first delivery”.

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.

## 3.28. 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.

## 3.29. Multiple personal mailboxes

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

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

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

The alternative approach is to pass all messages through a single .forward file, which must be a filter file in order to distinguish between the different cases by referencing the variables `$local_part_prefix` or `$local_part_suffix`, as in the final example in section 3.31 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 3.26) includes prefixes and suffixes in its checking.

## 3.30. 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.

## 3.31. Examples of Exim 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

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