Storing in different folder upon (envelope) sender:

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

Storing in different folder upon destination address:

require ["fileinto", "vnd.dovecot.filter"];
if address :is "to" "MyAlias1@rigacci.org" {
    fileinto "ToAlias1";
}

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 means that the message is not completed by the redirect action; eventually the keep action will be finally executed.

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

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).

require ["fileinto", "vnd.dovecot.filter", "envelope", "editheader"];
deleteheader "from";
addheader "From" "user1@forwarding.domain";
redirect "user2@forward.destination.domain";

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:

plugin {
  ...
  sieve_extensions = +vnd.dovecot.filter +editheader
  ...
}

require ["fileinto", "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";
}

The execute command is not considered a final action (which “consumes” the message), so the keep action is eventually taken.

• Pigeonhole Sieve: Extprograms Plugin
• GNU Mailutils Sieve