Looks like the SMTP-server does not accept the authentication. Just check the ./config/main.inc.php for the following options:
// SMTP username (if required) if you use %u as the username RoundCube
// will use the current username for login
$rcmail_config['smtp_user'] = '';
// SMTP password (if required) if you use %p as the password RoundCube
// will use the current user's password for login
$rcmail_config['smtp_pass'] = '';
// SMTP AUTH type (DIGEST-MD5, CRAM-MD5, LOGIN, PLAIN or empty to use
// best server supported one)
$rcmail_config['smtp_auth_type'] = '';