[9.x] Add Symfony Mailer to Upgrade Guide

[Jubeki]

There are probably lots of things to edit in this PR. I mostly copied and formatted the content of the laravel/framework#38481 PR description.

I also added some small code snippets and changed the order of some things.

I also was not sure at what level the headings need to be.

[taylorotwell]

We will perhaps need an example of this.

[Jubeki]

There were only Methods which were not documented by Laravel because, Laravel already used from, subject, cc` like the Symfony Mailer. There should probably not be many changes needed.

[taylorotwell]

Will likely need an actual code example of this.

[Jubeki]

I only have the verify_peer option in mind. Anything else which should be added as a code example?

[taylorotwell]

May need a list of these, plus link to the Symfony Mailer documentation where they can read about the public methods available on a Symfony Mail message, etc.

[Jubeki]

I looked at the PR for the framework and searched for all methods which where renamed.

Do you want to keep methods in the upgrade guide which are probably not used?

• Mailer::getSymfonyTransport()
• Mailer::setSymfonyTransport(TransportInterface $transport)
• MailManager::createSymfonyTransport($config)

[driesvints]

Thanks @Jubeki. Some minor changes are needed here.

[Jubeki]

Thanks @driesvints I reviewed and committed everything you wanted to change.

[taylorotwell]

Marking this as draft until I get closer to writing 9.0 upgrade guide.

[taylorotwell]

What kind of exception?

[Jubeki]

@taylorotwell
A TransportExceptionInterface will be thrown if the email could not be send.
https://symfony.com/doc/current/mailer.html#handling-sending-failures

The failed recipients are an artifact of Swift_RfcComplianceException, thus holds the invalid address (you can validate this yourself as well, before sending)
symfony/symfony#33812

It would probably be better to word it something like this:

Suggested change

	It's no longer possible to get a list of failed recipients. Instead, if an email is failed to send, and exception will be thrown.
	It's no longer possible to get a list of failed recipients. Instead, you need to validate the emails yourself before sending.
	Furthermore to validate if an email was really send you need to check this with your E-Mail Provider. For example through webhooks.

[taylorotwell]

I have rewritten this and merged it to the current master branch of the documentation. Thank you for your help.

[driesvints]

Thanks @Jubeki !