====== 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>

===== Recipient =====

Storing messages in different folder upon destination address:

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

===== 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]]**