SMTP error 535 authentication failed in roundcube OR SMTP Error (435): Authentication failed

If you face SMTP error 535 Authentication Failed while sending an email from round cube, then you can check the following things.

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

and change

$rcmail_config['smtp_user'] = '%u';
to
$rcmail_config['smtp_user'] = '';

Save the file

FIXED....