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’] = ”;
to
$rcmail_config[‘smtp_user’] = ”;
Save the file
FIXED….
