Hello Reader's if you want to write a custom code for making AJAX hit then you should choose for the best path for that, Open the file admin-ajax.php All ajax request should be made there.

On in first step open the template file and write the code below:-

<script type="text/javascript">
var ajaxurl = '<?php echo admin_url('admin-ajax.php'); ?>';
</script>

Now open the Javascript File and wite the code below:-

$.ajax({
        url: ajaxurl,    
        type: "POST",
        cache: false,
        data: data + '&action=sendmail' // or you can define the function
});

Now open the function.php file and make this function:-

function send_my_mail(){
#do your stuff
}

add_action('wp_ajax_sendmail', 'send_my_mail');
add_action('wp_ajax_nopriv_sendmail', 'send_my_mail');