function mymodule_form_alter(&$form, &$form_state, $form_id) {
    $form['actions']['submit']['#submit'][]='mymodule_submit';
}

function mymodule_submit($form, &$form_state)
{
    // Prepare vals
    $vals = $form_state['input']['submitted'];

    // Submit!
    $return = drupal_http_request('http://www.foreigndomain.com/?'.http_build_query($vals));
    if (strrpos(strip_tags($return->data),'Lead Received') !== false) {
        drupal_set_message(t('Information successfully sent.'));
    } else {
        form_set_error('form', strip_tags($return->data));
        $form_state['redirect'] = FALSE;
    }
}