title: Get a mail application to recognize a Reply-To email php
body: <p>I can't figure out how to get a mail application (except google mail) to recognize that an email was sent as a "Reply-To" and have those emails grouped together as one list of sent and replied emails.
  <br /></p>
  
  <p>For example, using php, if I use 
  <br /></p>
  
  <pre><code>$header = "From: Testing &lt;email@mail.com&gt;\r\n" .
                  "Reply-To: email@mail.com\r\n" . 
                  "X-Mailer: PHP/" . phpversion();
  
  $to = "email@mail.com";
  
  $message = "This is a reply";
  $subject = "test 123";  
  $success = mail($to, $subject, $message, $header);
  </code></pre>
  
  <p><br /> And send this twice, I get two separate emails. Instead of one email composed of the two emails.
  <br /> Is there a way to have them both grouped together as one email replied to another, or am I doing something wrong?
  <br />I've read the php mail() documentation and multiple webpages explaining how php mail works and still can't get the emails to reply to each other.
  <br />
  <br />
  Thank you for your time and help!</p>