app/bundles/EmailBundle/Mailer/Transport/InvalidTransport.php

<?php

declare(strict_types=1);

namespace Mautic\EmailBundle\Mailer\Transport;

use Symfony\Component\Mailer\Envelope;
use Symfony\Component\Mailer\Exception\TransportException;
use Symfony\Component\Mailer\SentMessage;
use Symfony\Component\Mailer\Transport\TransportInterface;
use Symfony\Component\Mime\RawMessage;

class InvalidTransport implements TransportInterface
{
    public function send(RawMessage $message, Envelope $envelope = null): ?SentMessage
    {
        throw new TransportException('Unknown DSN scheme. Please make sure the mailer DSN is configured properly.');
    }

    public function __toString(): string
    {
        return 'invalid://';
    }
}