You have requested a non-existent service "Symfony\Component\Mailer\Transport".

Thanks well spotted.

Calling Transport::fromDsn creates a new Transport object based on the `getDefaultFactories`.

What we actually want to use are the Drupal configured transport services, which exist in $MultiTransport->transportFactory. This will pick up all services tagged mailer.transport_factory including the SendmailCommandValidationTransportFactory decoration. Maybe we could add MultiTransport::validateDsn() to validate using this collection?

@rodrigoaguilera Please feel free to take a look and let me know if it works for you. Otherwise I'll check in before I make the next release.

I checked out fa24d4c6407eb0bacaf08099be868b938717e6e6 and succesfully created a transport with the interface.
The code look good. I don't really use the architecture with the tagged services but I guess is important for flexibility opening the door for things like decorating.

Great thanks