Validate system_send_email_action form submissions.
function system_send_email_action_validate($form, $form_state) {
$form_values = $form_state['values'];
// Validate the configuration form.
if (!valid_email_address($form_values['recipient']) && strpos($form_values['recipient'], ':mail') === FALSE) {
// We want the literal %author placeholder to be emphasized in the error message.
form_set_error('recipient', t('Enter a valid email address or use a token e-mail address such as %author.', array(
'%author' => '[node:author:mail]',
)));
}
}