Максим Горский,
текст нельзя стереть
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
window.addEventListener('DOMContentLoaded', function() {
var email = document.querySelector('.email'),
str = 'Email: ';
email.addEventListener('input', function() {
this.value = str + this.value.slice(str.length);
});
});
</script>
</head>
<body>
<input type="text" class="email" value="Email: test@mail.ru" placeholder="">
</body>
</html>