<?php
/**
 * @var MailAccount $mail_account
 */
$c = new Criteria();
$c->add(MailAccountPeer::IS_DEFAULT, true);
$is_default = $mail_account->isNew() && !MailAccountPeer::doCount($c);
$disabled = $mail_account->getIsDefault() || $is_default;
if ($is_default)
{
   $mail_account->setIsDefault($is_default);
}
?>
<?php echo st_admin_checkbox_tag('mail_account[is_default]', 1, $mail_account->getIsDefault(), array(
   'disabled' => $disabled,
)); ?> 

<?php if ($disabled) : ?>
    <?php echo input_hidden_tag('mail_account[is_default]', 1) ?>
<?php endif ?>