Regarding:
$config['smtp_server'] = '';

The default.inc.php config file notes that:

// SMTP server host (for sending mails).
// To use SSL/TLS connection, enter hostname with prefix ssl:// or tls://
// If left blank, the PHP mail() function is used

Sending messages are slow on my installation when sent via RoundCube (both RoundCube and QMail are on the same server). I'm wondering if supplying actual SMTP server information will have any impact on this.

Has anyone had experience in this area?

Thanks,
Denny