I'm using PHPMailer to send emails to support when our server is pinged with updates (usually related to payments). 

I'm trying to get related emails to display as a Gmail conversation to make it easier to support staff to follow previous updates/replies. I had originally assumed that it was based on subject, but that doesn't seem to make a difference.

My mailer code:

```
$mail               = new PHPMailer;                        // create a new instance
$mail->isSMTP();                                            // set that we're using stmp
$mail->CharSet      = 'UTF-8';                              // make sure it's utf-8 encoded
$mail->Host         = 'smtp.gmail.com';                     // the hostname of the mail server
$mail->Port         = 587;                                  // set the smtp port number (587 for authenticated TLS)
$mail->SMTPSecure   = 'tls';                                // set the encryption to use, ssl (deprecated) or tls
$mail->SMTPAuth     = true;                                 // should we use smtp authentication?
$mail->Username     = MY_EMAIL_LOGIN;                       // the user name for the smtp authentication
$mail->Password     = MY_EMAIL_PASSWORD;                    // the password for smtp authentication
$mail->wordWrap     = 70;                                   // make sure we've no lines longer than 70 chars
$mail->Subject      = "[Payment] - Player {$payment->user->name} ({$payment->user->id}) - Payment ID {$payment->id}";
$mail->Body         = $htmlBody;                            // our html body
$mail->AltBody      = $plainBody;                           // our fallback, plain-text body
$mail->setFrom( SUPPORT_EMAIL, 'Support' );                 // who this is from
$mail->addReplyTo( SUPPORT_EMAIL, 'Support' );              // who we can reply to
$mail->addAddress( SUPPORT_EMAIL );                         // who we're sending it to
$mail->isHTML( true );                                      // is this a html formatted email?
if( !$mail->send() )
    error_log( "Can't send an email to support about payment {$payment->id} for user {$payment->user->id}" );

```

If I get 2 emails from the same user relating to the same payment (so the same subject), what I want it to come in as:

```
+-----------------------------------------------------------------------------+
| Support (2)       | [Payment] Player foo (123456789) - Payment ID 123456789 |
+-----------------------------------------------------------------------------+

```

What it's actually coming in as:

```
+-----------------------------------------------------------------------------+
| Support           | [Payment] Player foo (123456789) - Payment ID 123456789 |
+-----------------------------------------------------------------------------+
| Support           | [Payment] Player foo (123456789) - Payment ID 123456789 |
+-----------------------------------------------------------------------------+

```

Am I missing something simple?