E-mail Validation In JavaScript

<script type="text/javascript">
    function validate_email(field, alerttxt) {
        with(field) {
            apos = value.indexOf("@");
            dotpos = value.lastIndexOf(".");
            if (apos < 1 || dotpos - apos < 2) {
                alert(alerttxt);
                return false;
            } else {
                return true;
            }
        }
    }

    function validate_form(thisform) {
        with(thisform) {
            if (validate_email(username, "Your UserName is Not Valid!") == false) {
                username.focus();
                return false;
            }
        }
    }
</script>