MAILER_DSN=gmail://myMail@gmail.com:myPassword@default

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

in vendor/symfony/mailer/Transport.php (line 156)
in vendor/symfony/mailer/Transport.php -> fromDsnObject (line 141)
in vendor/symfony/mailer/Transport.php -> parseDsn (line 93)
in vendor/symfony/mailer/Transport.php -> fromString (line 85)

    public function fromDsnObject(Dsn $dsn): TransportInterface
    {
        foreach ($this->factories as $factory) {
            if ($factory->supports($dsn)) {
                return $factory->create($dsn);
            }
        }

        throw new UnsupportedSchemeException($dsn);
    }