[ submit.php line 27 ]

$user_count = $wpdb->get_var("SELECT COUNT(*) FROM $table_email where email ='$_POST[email_add]' and id_lista = '$_POST[lista]';");

As you can see, $_POST[lista] parameter is nor validated neither escaped, so you can blind sql inject it using $user_count for the
boolean condition checking :

[ submit.php line 29 ]

if($user_count>0) :
  $errore_presente = "<div class="error">".__('email address already present', 'sendit')."</div>";
  die($errore_presente);

---
PoC
---

POST:

email_add = some.random.regexp.valid.email@domain.ltd
lista = BLIND SQL INJECTION HERE