Relating email replies to original email (e-mail thread) with Sendgrid

Question

I'm sending e-mails with Sendgrid. When the recipients reply to the mails, Sendgrid will parse the reply and POST to my webhook.

How can I relate the reply to the original message, so that I have a message thread?

I'm thinking that this may be possible using X-SMTPAPI headers and unique_args.

Update: I think this is how I did it (but it was a while ago)

 $arguments = ['unique_args' => [
           'sentMessage_id' => $this->sentMessage->id,
           'tenant_id' => \app('currentTenant')->id,
       ]];
       $header = \json_encode($arguments);

       $this->withSwiftMessage(function ($message) use ($header) {
           $thread = $this->sentMessage->message->thread;

           $headers = $message->getHeaders();
           $headers->addTextHeader('X-SMTPAPI', $header);
           $headers->addTextHeader('References', "<{$thread->first_mail_reference}>");