function validate() {
    var valid = true, output = '';

    if (document.mailform.name.value.length == 0) {
        valid = false;
        output = 'Please Enter a Name\n';
    }
    if (document.mailform.address.value.length == 0) {
        valid = false;
        output += 'Please Enter an Address';
    }

    if (!valid)
        alert(output);

    return valid;
}

<!-- <FORM NAME="mailform" onSubmit="return validate()"> -->!