In the /wp-content/plugins/newsletter/widget.php file on line 38 it should say <input class="newsletter-email" type="text" instead of <input class="newsletter-email" type="email". There is no email type for input elements.
Hi, the email type is correct, it’s the new HTML5 type. It works even on old browser since the input field defaults to text type when not specified or when unrecognized.