Ability to use custom transports with symfony/mailer

I don't understand these test failures, if anyone could help explain them I would be much appreciated!

Dependency injection has been left out intentionally from the original issue (see #3165762-30: Add symfony/mailer into core → for the reasoning). There is an open issue which tries to introduce the services properly in 📌 [PP-1] Add symfony mailer transports to DIC Postponed (that doesn't currently touch the experimental mail plugin though).

My plan was to add the plugin to give people something to play with. I wouldn't recommend to extend the experimental plugin any further, or even use it in production. Instead, I suggest that we define and implement a new API based on the Symfony mailer component and then deprecate the old mail plugin system (see also #1803948-81: [META] Adopt the symfony mailer component → for more details).

Would you accept a middle ground? Perhaps a TransportFactoryManager that just collects tagged transports, but doesn't create the Symfony Transport object?

I'm not the one to decide. And my plan (or better said my intention) doesn't count anything unless it gets traction in the community. I recently tagged 📌 [PP-1] Add symfony mailer transports to DIC Postponed with Needs framework manager review in the hope to gather some additional feedback from those folks who will eventually have to weight in on the approach.

This will eventually need framework manager here too.

I created a new merge request that is more incremental, and I think should solve my problem without having to add the Transport to the DI container.
https://git.drupalcode.org/project/drupal/-/merge_requests/5847

Please take a look and let me know what you think!

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

This does not mean that the patch needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

Rebased

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

This does not mean that the patch needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.