<html>
<head>
<meta charset="utf-8">
</head>
<body>
<input id="email" type="email" value="">
<button id="sendButton" disabled>Отправить</button>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
$('#email').keyup(function(){
$('#sendButton').prop('disabled', $(this).val().length<=5);
});
</script>
</body>
</html>