title: How to send gmail thread id in reply mail using PhpMailer
body: <p>I am using PHPMailer for sending and replying mail. Here i am facing issue is the reply thread mail is not linked with existing group. It will send separate mail. How can i send it with existing group. I am having thread id. Here is my code.</p>
  
  <pre><code>require_once 'plugins/PHPMailer/src/PHPMailer.php';
  require_once 'plugins/PHPMailer/src/Exception.php';
  require_once 'plugins/PHPMailer/src/SMTP.php';
  
  $mail = new PHPMailer(true);
  $mail-&gt;Username   = GMAIL_MAIL;
  $mail-&gt;Password   = GMAIL_PWD;                               // SMTP password
  $mail-&gt;SMTPSecure = 'tls';                                  
  $mail-&gt;Port       = 587;                                        $mail-&gt;setFrom(MAIL_FROM, '');
  $mail-&gt;addAddress($request['mail_to'], $request['mail_to']);     
  $mail-&gt;addReplyTo(MAIL_FROM, MAIL_FROM);
  $mail-&gt;addCustomHeader( 'In-Reply-To', '&lt;' . MAIL_FROM . '&gt;' );
  $mail-&gt;addCustomHeader( 'X-GM-THRID', '' . $request['thread_id']. '' );
  $mail-&gt;addCustomHeader('References', '[' . $request['thread_id'] . ']');
  </code></pre>