# grep password roundcubemail/config/main.inc.php
$rcmail_config['plugins'] = array('password');
# vim roundcubemail/plugins/password/config.inc.php
$rcmail_config['password_db_dsn'] = 'mysql://postfixadmin:passw0rd@localhost/postfix';
$rcmail_config['password_query'] = 'UPDATE mailbox SET password=%c WHERE username=%u';
thanks so much