Laravel Mailable that sends a custom id to reference webhook to Mailgun

verifyMailableWithWebhookID.php

<?php
/**
 * Build the message.
 *
 * @return $this
 */
public function build()
{
    return $this->markdown('emails.email-verification')
        ->subject('verify your email address')
        ->withSwiftMessage(function ($message) {
            $message->getHeaders()
              ->addTextHeader('X-Mailgun-Variables', json_encode([
                'type' => 'verification',
                'id'   => 'our_unique_id'
            ]));
        });
}