Description
In some cases, you may need to get SentMessage information after send.
As the docs say SentMessage generates by TransportInterface and NOT available through the Mailer by itself.
The reason for this is understandable - Mailer can't just return SentMessage as it can work async (through the Messenger)
I've seen related issues and rejected PR's (breaking BC), but still believe that Symfony should provide better DX in this case.

Example
In my case, I needed to get the Mailgun mail id to track it after sending it.
I ended up with this workaround - not brilliant as DX.
Later I faced another solution - to use directly TransportInformation, but again, it's not what you expecting from Mailer.

Proposal
As a developer, I expect to deal with Mailer as the main Facade component, not Transport, therefore I think this PR makes sense for the next major version where we can break BC and make return type ?SentMessage for sync sending.

As for now, we can make a new Event called MessageSentEvent that will provide SentMessage for minor version updates, and still useful for async mode.

<?php

namespace use Symfony\Component\Mailer\Event;

use Symfony\Component\Mailer\SentMessage;

class MessageSentEvent
{
    private $sentMessage;


    public function __construct(SentMessage $sentMessage)
    {
        $this->sentMessage = $sentMessage;
    }

    public function getSentMessage(): SentMessage
    {
        return $this->sentMessage;
    }
}

And dispatch this Event after transport sends a message:

1. In Mailer::send method for sync mode:

...
        if (null === $this->bus) {
            $sentMessage=$this->transport->send($message, $envelope);
            $event = new MessageSentEvent($sentMessage);
            $this->dispatcher->dispatch($event);

            return;
        }
...

2. In MessageHandler::invoke for async mode:

...
$sentMessage=$this->transport->send($message->getMessage(), $message->getEnvelope());
$event = new MessageSentEvent($sentMessage);
$this->dispatcher->dispatch($event);

return $sentMessage;
...

After that, it becomes easy to subscribe to this event and process SentMessage information.