How to send the checkbox value to email

First you need to add a class for all checkbox as having same id for multiple items is not recommended

<input type="checkbox" class="new_class" id='haet_mail_test_address' name="ckboxs[]"  value="".$emails[$i]["user_email']."'>

Now we to change the way you are getting the values from checkbox

Change

var checkbox = $('#haet_mail_test_address').val();

To

var checkbox = $(".new_class:checkbox:checked").map(function(){
  return $(this).val();
}).get();

console.log(checkbox); // check if you are getting the values here

Now in “checkbox” you will get an array of selected checkbox values. Now you can pass it to the ajax function as data