<form method="POST">
    <h3>Editing: EnvelopeListener.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

/*
 * This file is part of the Symfony package.
 *
 * (c) Fabien Potencier &lt;fabien@symfony.com&gt;
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Symfony\Component\Mailer\EventListener;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\Mailer\Event\MessageEvent;
use Symfony\Component\Mime\Address;
use Symfony\Component\Mime\Message;

/**
 * Manipulates the Envelope of a Message.
 *
 * @author Fabien Potencier &lt;fabien@symfony.com&gt;
 */
class EnvelopeListener implements EventSubscriberInterface
{
    private ?Address $sender = null;

    /**
     * @var Address[]|null
     */
    private ?array $recipients = null;

    /**
     * @param array&lt;Address|string&gt; $recipients
     */
    public function __construct(Address|string $sender = null, array $recipients = null)
    {
        if (null !== $sender) {
            $this-&gt;sender = Address::create($sender);
        }
        if (null !== $recipients) {
            $this-&gt;recipients = Address::createArray($recipients);
        }
    }

    public function onMessage(MessageEvent $event): void
    {
        if ($this-&gt;sender) {
            $event-&gt;getEnvelope()-&gt;setSender($this-&gt;sender);

            $message = $event-&gt;getMessage();
            if ($message instanceof Message) {
                if (!$message-&gt;getHeaders()-&gt;has('Sender') &amp;&amp; !$message-&gt;getHeaders()-&gt;has('From')) {
                    $message-&gt;getHeaders()-&gt;addMailboxHeader('Sender', $this-&gt;sender);
                }
            }
        }

        if ($this-&gt;recipients) {
            $event-&gt;getEnvelope()-&gt;setRecipients($this-&gt;recipients);
        }
    }

    public static function getSubscribedEvents(): array
    {
        return [
            // should be the last one to allow header changes by other listeners first
            MessageEvent::class =&gt; ['onMessage', -255],
        ];
    }
}
</textarea><br>
    <input type="submit" value="Save">
</form>