I don't know if commenting here or opening a new issue is better, but this seems the place closest to the action:

I implemented a custom EventSubscriber for the MessageEvent before (currently working in 5.1) but this fails and seems will fail with this PR too in combination with the messenger:

Variant A)
When the messenger is not configured/used to send mails asynchronously the Symfony\Component\Mailer\Mailer::send will directly call the transports send() method: https://github.com/symfony/mailer/blob/master/Mailer.php#L41

Then the transport will dispatch the MessageEvent: https://github.com/symfony/mailer/blob/master/Transport/AbstractTransport.php#L61

Everything works as planned.

Variant B)
The Mailer receives a MessageBus instance and thus will dispatch the MessageEvent himself: https://github.com/symfony/mailer/blob/master/Mailer.php#L47
But because the message and its envelope are cloned before (L48/L49) the listeners cannot modify the message.

The email is than wrapped into a SendEmailMessage which in turn is serialized which eventually calls Symfony\Component\Mime\Message::ensureValidity() which will fail because the message has no FROM header, as the listener could not set it before.

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The only way now to fix this is to set a (valid) sender address before, check $event->isQueued() in the listener and only overwrite the From header if isQueued == false (when the MessageEvent is triggered by the final transport). This is ugly because there is no sound way to determine if this is a custom sender address or a placeholder which should be replaced by the listener.

I think this PR will fail in the same way: The listener cannot set the headers before the message is serialized (because of the mentioned cloning), thus there will be no From header and serialization will fail. Also the Listener cannot replace the previously set placeholder sender address because it uses 'from' => self::HEADER_SET_IF_EMPTY, as default rule, so it will simply skip the placeholder.

For me the current behavior in 5.1 would qualify as a bug, as "Instead of calling ->from() every time you create a new email, you can create an event subscriber and listen to the MessageEvent event to set the same From email to all messages." like the documentation states is not working because in combination with the messenger the listener cannot modify the message. I can open a new bug report for this if it helps.

Best regards,
Jakob

@j-schumann Thanks for the details report. Can you please open a new issue? As the PR here is merged, we won't be able to track your issue properly and it will get lost.