private function buildEmailPayload( $type, $to, $templateId, array $personalisation, $reference, $emailReplyToId = NULL ) {

        $payload = $this->buildPayload( $type, $to, $templateId, $personalisation, $reference );

        if ( isset($emailReplyToId) && $emailReplyToId != '' ) {
            $payload['email_reply_to_id'] = $emailReplyToId;
        }

        return $payload;

    }