gistfile1.txt

Validation.add(
    'validate_unique_email',
    'This email is already in use! Please log in in order to proceed',
    function(value){

        var url = validationUniqueEmail + '?email=' + encodeURIComponent(value);
        var ok = false;

        new Ajax.Request(url, {
            method: 'get',
            asynchronous: false,
            onSuccess: function(transport) {

                var obj = response = eval('(' + transport.responseText + ')');
                validateTrueEmailMsg = obj.status_desc;

                if (obj.ok === false) {
                    Validation.get('validate-email').error = validateTrueEmailMsg;
                    ok = false;
                } else {
                    ok = true; /* return true or false */
                }
            },
            onComplete: function() {
                if ($('advice-validate-email-email')) {
                    $('advice-validate-email-email').remove();
                }
                if ($('advice-validate-email-email_address')) {
                    $('advice-validate-email-email_address').remove();
                }
                if ($('advice-validate-email-billing:email')) {
                    $('advice-validate-email-billing:email').remove();
                }
                if ($('advice-validate-email-shipping:email')) {
                    $('advice-validate-email-shipping:email').remove();
                }
                if ($('advice-validate-email-_accountemail')) {
                    $('advice-validate-email-_accountemail').remove();
                }
            }
        });
        return ok;
    }
);