Installation

Install dovecot-pigeonhole:

apk add dovecot-pigeonhole-plugin

Configuration

Enable the sieve plugin for the lmtp protocol in /etc/dovecot/conf.d/20-lmtp.conf:

protocol lmtp {
  # Space separated list of plugins to load (default is global mail_plugins).
  mail_plugins = $mail_plugins sieve
}

Enable the sieve plugin globally in /etc/dovecot/conf.d/20-managesieve.conf:

protocols = $protocols sieve

Allow TCP traffic on port 4190:

ufw allow 4190/tcp

Reload the Dovecot configuration to enable the pigeonhole plugin:

/etc/init.d/dovecot reload

Check if Dovecot is listening on the Sieve port (4190):

netstat -anp | grep dovecot

Thunderbird Add-on

Download and install the Sieve Add-on for Thunderbird. If you go to Tools > Sieve Message Filters, Thunderbird should open the page that lists your server connections. You should be able to connect to the Sieve server and manage the sieve scripts for your accounts.