Rouncube hangs while sending message

Whenever you compose email and click send, it just never sends the message. A circle keeps rotating at the top saying Sending.

Solution is:

Login to server with root user and made following changes.

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

and serach for following line

$rcmail_config['smtp_user'] = '%u';
Replace it with
$rcmail_config['smtp_user'] = '';