mensagem de erro:

[30-Jan-2009 14:07:11] Invalid response code received from server (421):
[30-Jan-2009 14:07:11] Invalid response code received from server (-1):
[30-Jan-2009 14:07:34 -0800] SMTP Error: SMTP error: Authentication failure: STARTTLS failed (Code: ) in /usr/local/cpanel/base/3rdparty/roundcube/program/steps/mail/func.inc on line 1248 (POST /3rdparty/roundcube/index.php?_task=mail&_action=send)
[30-Jan-2009 14:10:29] STARTTLS failed ():

Solução:

editar /usr/local/cpanel/base/3rdparty/roundcube/config/main.inc.php

atual
// SMTP username (if required) if you use %u as the username RoundCube
// will use the current username for login
$rcmail_config['smtp_user'] = '%u';

mudar para

// SMTP username (if required) if you use %u as the username RoundCube
// will use the current username for login
$rcmail_config['smtp_user'] = '';

E pronto Solução para o erro.