Error : When the click the send button, it starts to sending but nothing happens, the roundcube shows only the process state.

Solution : Then you need to follows the steps

If the server is cpanel server then open the file /usr/local/cpanel/base/3rdparty/roundcube/config/main.inc.php and search the
$rcmail_config['smtp_user'] = ‘%u’;
and replace it with
$rcmail_config['smtp_user'] = ‘ ‘;
Save the file and try to send emails.
Done