[RCU] mail_domain ignored?

My users log into Roundcube 1.0.2 using only their username (as opposed to username@domain). I further configured Roundcube to connect to the IMAP server at localhost. I have tried to set mail_domain, but outgoing mail still contains user@localhost in the from address field.

My config.inc.php is:

<?php

/* Local configuration for Roundcube Webmail */

$config['db_dsnw'] = ...

$config['smtp_log'] = ...

$config['default_host'] = 'localhost';

$config['smtp_server'] = 'localhost';

$config['mail_domain'] = 'flyn.org'; // Ignored?

$config['support_url'] = ...

$config['log_dir'] = ...

$config['temp_dir'] = ...

$config['des_key'] = ...

$config['plugins'] = array('http_authentication', 'movespam');

$config['movespam_key'] = 'X-Bogosity';

$config['movespam_value'] = 'Spam';

$config['movespam_seen'] = false;

Am I doing something wrong?

Thank you,