I'm assuming that you have actually filled out both the subject and message fields? Did this just start happening or did you change something? What version of vb and php are you using?
You can try temporarily disabling all plugins by adding this line to your includes/config.php file, just under the <?php line at the top:
Code:
define('DISABLE_HOOKS', 1);
You can also try creating a new style with no parent, then see if you still have the problem when using that style.