IIUC, you put the entire email in the message envelope and you dispatch it?
If yes, I would recommend to compute the entire mail payload in the handler instead, and passing to the handler just the bare data :)

IIUC, ...

i am sorry but i dont know what you mean with IIUC.

... you put the entire email in the message envelope and you dispatch it?

do i have a choice? I just send the mail. What then is happen is done by Mailer.

IIUC :> If I Understand Correctly

i do not know what mailer does from its side on async logic, i send async email like i've described to you :)

I have the same problem on Symfony 4.* and 5.*

$email = (new TemplatedEmail())
->from(new Address('[email protected]', 'Company'))
->to($user->getEmail())
->priority(TemplatedEmail::PRIORITY_HIGH)
->subject($translator->trans('Your password reset request'))
->htmlTemplate('reset_password/email.html.twig')
->context([
    'resetToken' => $resetToken,
]);

$mailer->send($email);

I've just run into this.

I would have expected that when I do a $mailer->send($templatedEmail), that the Mailer would render the TemplatedEmail before adding the email to the async transport.

Instead, the Mailer is serializing the TemplatedEmail and dispatching it to the queue (with the containing Envelope and Stamps) and then when the message is consumed, the Mailer is deserializing the TemplatedEmail and rendering before sending.

@gubler Same issue here. I expected too that rendering is done before sending to the async transport.
Workaround:

# services.yaml
Symfony\Component\Mime\BodyRendererInterface:
        alias: twig.mime_body_renderer

# Controller/TestController.php
public function myAction(MailerInterface $mailer, BodyRendererInterface $bodyRenderer): Response
{
    $email = (new TemplatedEmail());
    $email->to('[email protected]');
    $email->htmlTemplate('emails/test.html.twig');
    $email->context(['foo' => 'bar']);
    $this->bodyRenderer->render($email);
    $this->mailer->send($email);
    new Response('OK');
}

Although my issue was not exactly the same, this issue helped me find out what the problem was.

Some properties in the template were missing. It was caused by a custom __serialize() function in an object passed to the context. So to be safe, only use really simple DTOs in the context.

I hope it can help someone.

Calling the body renderer seems not be enough in all situations. When using custom serialize functions it may be happen that the fingerprint differs between the first call to the body renderer and the call that is made by the message handler. This will lead into a second renderer which will fail in our case.

Extending above work a round with some extra code to convert een templated email into a plain email seems to work.

    $email = (new TemplatedEmail())
        ->to(new Address($to))
        ->subject($subject)
        ->htmlTemplate($template)
        ->context($context)
    ;
    $bodyRenderer->render($email);

    $plainEmail = new Email();
    $plainEmail->setHeaders(clone $email->getHeaders());
    $plainEmail->html($email->getHtmlBody());
    $plainEmail->text($email->getTextBody());

    $mailer->send($plainEmail);

The mailer renders the email in the async handler to get "better" headers (like a Date header that reflects when the email was sent).
When sending an email async, you must indeed send only data that can be serialized (there is no way around it). The other proper way is to render the email yourself via the body renderer as mentioned in one of the comments above (with some caveats like for the Date header value mentioned above).

See #47075 for an easier way to render emails before sending them in 6.2
And symfony/symfony-docs#17065 for the doc PR that makes things hopefully more clear.