AfterMailerInitializationEvent 

Example 

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

services:
  # Place here the default dependency injection configuration

  MyVendor\MyExtension\Mail\EventListener\MyEventListener:
    tags:
      - name: event.listener
        identifier: 'my-extension/after-mailer-initialization'

Read how to configure dependency injection in extensions.

An example listener, which hooks into the Mailer API to modify mailer settings to not send any emails ("null mailer"), could look like this:

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\Mail\EventListener;

use TYPO3\CMS\Core\Mail\Event\AfterMailerInitializationEvent;

final class MyEventListener
{
    public function __invoke(AfterMailerInitializationEvent $event): void
    {
        $event->getMailer()->injectMailSettings(['transport' => 'null']);
    }
}

API 

class AfterMailerInitializationEvent

Fully qualified name

\TYPO3\CMS\Core\Mail\Event\AfterMailerInitializationEvent

This event is fired once a new Mailer is instantiated with specific transport settings.

So it is possible to add custom mailing settings.

getMailer ( )

Returns

SymfonyComponentMailerMailerInterface