// config/packages/mailer.yaml

framework:
    mailer:
        dsn: '%env(MAILER_DSN)%'

services:
    mailer.transport_factory.smtp:
        class: App\Mail\Transport\EsmtpTransportFactory
        parent: mailer.transport_factory.abstract
        tags:
            - { name: mailer.transport_factory, priority: -100 }

// src/App/Mail/Transport/EsmtpTransportFactory.php

<?php

namespace App\Mail\Transport;

use Symfony\Component\Mailer\Transport\AbstractTransportFactory;
use Symfony\Component\Mailer\Transport\Dsn;
use Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport;
use Symfony\Component\Mailer\Transport\TransportInterface;

class EsmtpTransportFactory extends AbstractTransportFactory
{
    public function create(Dsn $dsn): TransportInterface
    {
        $tls = 'smtps' === $dsn->getScheme() ? true : null;
        $port = $dsn->getPort(0);
        $host = $dsn->getHost();

        $transport = new EsmtpTransport($host, $port, $tls, $this->dispatcher, $this->logger);
        $transport->setPingThreshold(10);

        if ($user = $dsn->getUser()) {
            $transport->setUsername($user);
        }

        if ($password = $dsn->getPassword()) {
            $transport->setPassword($password);
        }

        return $transport;
    }

    protected function getSupportedSchemes(): array
    {
        return ['smtp', 'smtps'];
    }
}