Symfony version(s) affected

5.3

Description

Symfony Mailer converts the email addresses domain to A-label in order to be compliant with servers that does not support the SMTPUTF8 flag but this conversion is not fully IDNA2008 compliant.

See #44091 that is the equivalent report for HttpClient.

This line should be fixed.

How to reproduce

$stream = new SocketStream();
$stream
    ->setHost($host)
    ->setPort($port)
    ->disableTls();
$transport = new SmtpTransport($stream);
$mailer = new Mailer($transport);
$email = (new Email())
    ->from($from)
    ->to($to)
    ->subject($subject)
    ->text($body);
$mailer->send($email);

If $to domain is fußball.test it should be converted to xn--fuball-cta.test instead of fussball.test.

Possible Solution

The following flags should be provided to be the most compliant possible with IDNA 2008:
IDNA_DEFAULT | IDNA_USE_STD3_RULES | IDNA_CHECK_BIDI | IDNA_CHECK_CONTEXTJ | IDNA_NONTRANSITIONAL_TO_ASCII

Additional Context

No response