Feature: #93689 - PSR-14 events on sending messages with Mailer 

Description 

TYPO3's MailerInterface implementation Mailer is used for sending messages, e.g. in the EXT:form email finishers. To allow further handling and manipulation of the message sending process, two new PSR-14 events have been introduced.

The BeforeMailerSentMessageEvent is dispatched before the message is sent by the mailer and can be used to manipulate the RawMessage and the Envelope. Usually an Email or FluidEmail instance is given as RawMessage. Additionally, the MailerInstance is given, which depending on the implementation - usually \TYPO3\CMS\Core\Mail\Mailer - contains the Transport object, which can be retrieved using the getTransport() method.

The AfterMailerSentMessageEvent is dispatched as soon as the message has been sent via the corresponding TransportInterface. The event receives the current MailerInstance, which, depending on the implementation - usually \TYPO3\CMS\Core\Mail\Mailer - contains the SentMessage object that can be retrieved using the getSentMessage() method.

Registration of the event listeners in your extension's Services.yaml:

MyVendor\MyPackage\EventListener\MailerSentMessageEventListener:
  tags:
    - name: event.listener
      identifier: 'my-package/modify-message'
      method: 'modifyMessage'
    - name: event.listener
      identifier: 'my-package/process-sent-message'
      method: 'processSentMessage'

The corresponding event listener class:

use Psr\Log\LoggerInterface;
use Symfony\Component\Mime\Address;
use Symfony\Component\Mime\Email;
use TYPO3\CMS\Core\Mail\Event\AfterMailerSentMessageEvent;
use TYPO3\CMS\Core\Mail\Event\BeforeMailerSentMessageEvent;
use TYPO3\CMS\Core\Mail\Mailer;

final class MailerSentMessageEventListener
{
    public function __construct(
        private readonly LoggerInterface $logger
    ) {
    }

    public function modifyMessage(BeforeMailerSentMessageEvent $event): void
    {
        $message = $event->getMessage();

        // If $message is an Email implementation, add an additional recipient
        if ($message instanceof Email) {
            $message->addCc(new Address('kasperYYYY@typo3.org'));
        }
    }

    public function processSentMessage(AfterMailerSentMessageEvent $event): void
    {
        $mailer = $event->getMailer();
        if ($mailer instanceof Mailer) {
            $sentMessage = $mailer->getSentMessage();
            if ($sentMessage !== null) {
                $this->logger->debug($sentMessage->getDebug());
            }
        }
    }
}

Impact 

With the new PSR-14 events, it's now possible to manipulate messages before they are sent by the mailer. Additionally, after the mailer has sent messages, further processing can be performed.