Zend FR

<?php
require_once 'Zend/Validate/Abstract.php';

require_once 'Users.php';

class Rx_Validate_EmailExists extends Zend_Validate_Abstract {

    const EXISTS = 'emailNotExists';

    protected $_messageTemplates = array(
        self::EXISTS => 'This email does not exist in our database'
    );

    public function isValid($value) {
        $this->_setValue((string)$value);

        $users = new Users();
        $exists = $users->getByEmail($this->_value);

        if (count($exists)>0) {
            return true;
        }

        $this->_error(self::EXISTS);
        return false;
    }
}