private static function getMessageMailerEvents(): MessageEvents
{
    if (!self::getClient()->getRequest()) {
        static::fail('Unable to make email assertions. Did you forget to make an HTTP request?');
    }

    if (!$logger = self::$container->get('mailer.logger_message_listener')) {
        static::fail('A client must have Mailer enabled to make email assertions. Did you forget to require symfony/mailer?');
    }

    return $logger->getEvents();
}

<?php

namespace App\Tests;

use Symfony\Bundle\FrameworkBundle\Console\Application;
use Symfony\Bundle\FrameworkBundle\Test\MailerAssertionsTrait;
use Symfony\Component\Console\Tester\CommandTester;

class MyCommandTest extends WebTestCase
{
    use MailerAssertionsTrait;

    public function testCommand(): void
    {
        $application = new Application(self::$kernel);
        $command = $application->find('app:my-command');
        $commandTester = new CommandTester($command);

        $commandTester->execute(['command' => $command->getName()]);

        self::assertEmailCount(1);
    }
}

private static function getMessageMailerEvents(): MessageEvents
{
    if (!$logger = self::$container->get('mailer.logger_message_listener')) {
        static::fail('A client must have Mailer enabled to make email assertions. Did you forget to require symfony/mailer?');
    }

    return $logger->getEvents();
}