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====== Filtering with Sieve ======

The **Debian 11 Bullseye** GNU/Linux installs the **Dovecot** mail suite version **2.3.13**, including the **Pigeonhole Sieve** extension **v0.5.13** (via the **dovecot-sieve** package).

The [[https://doc.dovecot.org/configuration_manual/sieve/pigeonhole_sieve_interpreter/|Pigeonhole Sieve Interpreter]] provides the //pipe//, //filter// and //execute// commands, where the //pipe// was the older one implemented (starting from v0.2). Currently the more modern //execute// command can be used instead of the //pipe// one.

The following recipes were tested into a **$HOME/.dovecot.sieve** file, i.e. the users's Sieve filter applied by the Dovecot Local Delivery Agent.

FIXME Document the timeout options.

===== Sender =====

Storing messages in different folder upon (envelope) sender:

<code>
require ["fileinto", "envelope"];
if envelope "from" "user1@rigacci.org" {
    fileinto "FromUser1";
}
</code>

Discard a message by sender:

<code>
if address :is "from" "MAILER-DAEMON@mail.rigacci.org" {
    discard;
    stop;
}
</code>

Same result, but use the generic //header// command instead of //address//:

<code>
if header :contains "From" "MAILER-DAEMON@mail.rigacci.org" {
    discard;
    stop;
}
</code>

===== Recipient =====

Storing messages in different folder upon destination address:

<code>
require ["fileinto"];
if address :is "to" "MyAlias1@rigacci.org" {
    fileinto "ToAlias1";
}
</code>

===== Body =====

Storing messages in different folder upon body content (this requires the [[https://datatracker.ietf.org/doc/html/rfc5173.html|Sieve Body Extension]], which is supported by the Dovecot Pigeonhole Sieve Interpreter):

<code>
require ["fileinto", "body"];
if body :contains  "Address Verification Required" {
    fileinto "Spam";
}
</code>

===== Forward and keep a local copy =====

The **redirect** built-in //action// can be modified by the **:copy** //tag// (which is a built-in extension, but it must enabled using the **require** statement). The '':copy'' tag means that the message is not completed by the ''redirect'' action; eventually the ''keep'' action will be finally executed.

<code>
require ["copy"];
# Forward the messages unconditionally and continue processing.
redirect :copy "niccolo@texnet.it";
</code>

===== Forward with From rewrite (to pass SPF check) =====

Rewrite the **From** header before forwarding, to ensure to pass the SPF checks at destination. **This hack alone does not work**, beacuse it rewrites only the From header, not the envelope sender (which is generally reported as //Return-Path//).

<code>
require ["editheader"];
deleteheader "from";
addheader "From" "user1@forwarding.domain";
redirect "user2@forward.destination.domain";
</code>

**NOTICE**: To use the //editheader// plugin you must enable it into Dovecot configuration file (in Debian edit **/etc/dovecot/conf.d/90-sieve.conf**), adding it into the **sieve_extensions** list:

<code>
plugin {
  ...
  sieve_extensions = +vnd.dovecot.filter +editheader
  ...
}
</code>

**NOTICE**: The **envelope sender** is reported as a **Return-Path** header on the recipient side, but changing the Return-Path header with a Dovecot Sieve rule is not equivalent to changing the envelope sender.

One noticeable recipient that checks the **envelope sender** is Gmail, so it is not sufficient to change the **From** header to bypass the SPF check and get the message delivered to a Gmail account. Fortunately Dovecot Sieve allows you to change the envelope sender: you have to do it into the system configuration file.

In Debian you can add it into the **/etc/dovecot/conf.d/90-sieve.conf** configuration file:

<file>
plugin {
  ...
  sieve_extensions = +vnd.dovecot.filter +vnd.dovecot.execute +editheader
  ...
  # Header fields must not exceed one kilobyte
  sieve_editheader_max_header_size = 4k
  # Specifies what envelope sender address is used for redirected messages.
  sieve_redirect_envelope_from = orig_recipient
  ...
}
</file>

The **sieve_redirect_envelope_from** option accepts several values, see the [[https://doc.dovecot.org/settings/pigeonhole/#pigeonhole_setting-sieve_redirect_envelope_from|pigeonhole manual]]:

^ sender  | The sender address is used (default).  |
^ recipient  | The final recipient address is used.  |
^ orig_recipient  | The original recipient is used.  |
^ user_email  | The user’s primary address is used. This is configured with the **sieve_user_email** setting. If that setting is not configured, ''user_email'' is equal to ''sender''.  |
^ postmaster  | The **postmaster_address** configured for LDA/LMTP.  |
^ <user@domain>  | Redirected messages are always sent from ''user@domain''. The angle brackets are mandatory. The null ''%%<>%%'' address is also supported.  |

It seems that Goole requires a SPF valid **envelope from**, but it is happier if you modify also the **From** header:


<file>
if anyof (
    header :contains "Subject" "Invio Bolletta Conto Contrattuale",
    address :is "to" "user@rigacci.org" ) {
    deleteheader "From";
    addheader :last "From" "Forward <noreply@rigacci.org>";
    redirect "user.name@gmail.com";
    keep;
}
</file>

===== Pipe a message to an external program =====

To enable the **execute** Sieve command using **Debian 11** you have to edit some configuration files contained into the **/etc/dovecot/conf.d/** directory. Set the **sieve_extension** option editing the file **90-sieve.conf**; in this example both the //filter// and //execute// extensions are loaded:

<file>
plugin {
   ...
   sieve_extensions = +vnd.dovecot.filter +vnd.dovecot.execute
   ...
}
</file>

Then you have to edit the **90-sieve-extprograms.conf** configuration file and define the **sieve_execute_socket_dir** and **sieve_execute_bin_dir** options:

<file>
plugin {
  ...
  sieve_execute_socket_dir = sieve-execute
  sieve_execute_bin_dir = /usr/local/lib/dovecot/sieve-execute
  ...
}
</file>

Only the executables contained into //sieve_execute_bin_dir// can be used as //execute// commands in sieve scripts.

The following example is used to pipe a received message to an external program, which acts as a gateway from the mail system to the SMS mobile network. Only messages originating from some addresses and containing a properly formatted subject are piped to the external program:

<code>
require ["vnd.dovecot.execute"];
# Specially crafted messages are piped to an SMS gateway.
if allof (
    anyof (
        address :is "from" "user1@rigacci.org",
        address :is "from" "user2@rigacci.org"),
    header :contains "Subject" "SMS" ) {
        execute :pipe "sms-gateway";
}
</code>

Here the **execute** //command// is called with the **:pipe** //tag//, this causes the execution of the external program much like the older ''pipe'' command.

The **execute** command is not considered a final action (which "consumes" the message), so the **keep** action is eventually taken, saving the message into the INBOX.

  * **[[https://doc.dovecot.org/configuration_manual/sieve/plugins/extprograms/|Pigeonhole Sieve: Extprograms Plugin]]**
  * **[[https://mailutils.org/manual/html_chapter/Sieve-Language.html|GNU Mailutils Sieve]]**

===== Selection Criteria =====

^ %%header :contains "subject" "string"%%  | The string is contained somewhere in the Subject header. Wildcards (e.g. **''%%*%%''** or **''%%?%%''** are not allowed).  |
^ %%header :matches "subject" "string"%%   | The string must match exactly the Subject header, you can use wildcards to extend the match, eg. **''%%*%%''** matches any sequence of characters, **''%%?%%''** matches only one character.  |
^ %%exists "X-Spam-Report"%%               | Check the existence of the specified header.   |


  * **[[https://en.wikipedia.org/wiki/Sieve_%28mail_filtering_language%29#Example|Sieve (mail filtering language) Example]]**
  * **[[https://doc.dovecot.org/2.3/configuration_manual/sieve/examples/|Sieve Examples]]**
  * **[[https://p5r.uk/blog/2011/sieve-tutorial.html|Sieve Tutorial]]**