For host security reason in email - is best for add from for send email with to ...
Find:
$validation_url = $script_url."/admin/index.php?goto=".urlencode("validateimages.php?action=validateimages");
$site_email->set_to($config['site_email']);
replace:
$validation_url = $script_url."/admin/index.php?goto=".urlencode("validateimages.php?action=validateimages");
$site_email->set_from(stripslashes(trim($config['site_email'])));
$site_email->set_to(stripslashes(trim($config['site_email'])));