title: PHP mail() working only on some domains
body: <p>I'm trying to send out emails with PHP using this code</p>
  
  <pre><code>$headers = "From: " . "from@myself.com" . "\r\n";
  $headers .= "MIME-Version: 1.0\r\n";
  $headers .= "Content-Type: text/html; charset=utf-8\r\n";
  
  mail($to, $subject, $content, $headers);
  </code></pre>
  
  <p>where <code>$to</code> is a valid email address and <code>$content</code> is the content of an html template.</p>
  
  <p>@gmail and @hotmail addresses both receive the emails correctly. @virgilio.it addresses doesn't receive them: what can be causing this? Some domains are accepting and displaying emails from my server: other don't. Why is this?</p>