[Answer]-Not returning ajax response?

PASS csrf_token .

Your AJAX response should be

var CSRF_TOKEN = document.getElementsByName('csrfmiddlewaretoken')[0].value
$.ajax({
          type:'POST',data:{email:emailstring, csrfmiddlewaretoken: CSRF_TOKEN },
          url:'/registration/ForgotUsername/',
          datatype:'json',
          success:function(data) {
              alert("email exists");
              alert(data);
              $('#ques').val(data);
       }

To achieve that in fast way, not the best you can add {% csrf_token %} into your form and pass the whole form in your ajax request by serializing it:

var yourForm = $("#your-form-id");

$.ajax({
    type:'POST',data:yourForm.serialize(),
    url:'/registration/ForgotUsername/',
    datatype:'json',
    success:function(data) {
        alert("email exists");
        alert(data);
        $('#ques').val(data);
    }
});

Serialization will convert the all form elements including csrf_token into desired data format.