The best way would be to code a plugin, I think you could use the message_sent hook Plugin_Hooks and some javascript to redirect to a "message sent" page.
Thanks for the reply.
$this->add_hook('message_sent', array($this, 'show_msgsent'));
public function show_msgsent($args)
{
$rcmail = rcmail::get_instance();
$rcmail->output->send("custompages.msgsent");
return $args;
}
This doesn't shows msgsent page, I couldn't understand where & how to use Js.
Please help.