jQuery validate - Check if email exists using remote request (ajax)

Question

I am using the 'remote' parameter validate jQuery to access a script that checks that the email exists or not. I checked with Firebug and the request is made, but I can not return and error message in the field. Below is my Javascript and PHP code:

Javascript:

mail: {
                required: true,
                email: true,
                remote: {
                    url: 'ajax/usuario.checar.mail.php',
                    type: 'POST'
                }
}

PHP:

if($checar > 0)
{
    // If e-mail exists, error message below
    echo "E-mail já cadastrado!";
    exit;
}

// If e-mail not exists, return true 
echo "true";