Send HTML email from my module
Solved that.
I added a line on my code i.e $message['headers']['Content-Type'] = 'text/html; charset=UTF-8;'

class invite_an_userMailSystem extends DefaultMailSystem {
  public function format(array $message) {
    $message['headers']['Content-Type'] = 'text/html; charset=UTF-8;';
    $message['body'] = implode("\n\n", $message['body']);
    $message['body'] = drupal_wrap_mail($message['body']);
    return $message;
  }
}


This is now working for me.. Yeppi :)