To install:
    1 - edit roundcube/config/main.inc.php
    edit/add the following config:

    $rcmail_config['plugins']
    add 'force_pass' to the array. if this array does not exist, add this:
    $rcmail_config['plugins'] = array('force_pass');

    2 - copy the directory force_pass to roundcube/plugins/

    3 - configure the plugin, edit the file
    roundcube/plugins/force_pass/config.inc.php

    edit this line:
    $rcmail_config['password_db_dsn']

    and this line:
    $rcmail_config['dayslimit']

    4 - edit the file roundcube/plugins/force_pass/savepass.php
    set the variable $config.

    5 - edit your postfix database, you will need a new column in the Mailbox
    table:
ALTER TABLE mailbox ADD pass_modifed DATETIME NOT NULL AFTER second_last_password;

    Note: You need to install the pf_password plugin with support to the
    pass_modified column.