I was wondering, there are several mailer services (mailjet, brevo, mailgun) that allow to design templates directly on their platform.
Currently the mailer component cannot manage this, it's mandatory to use twig as rendering engine.

What do you think about adding this capacity with a new class like ExternalTemplatedEmail with a new property "templateId".

If you think it's a good idea and can be integrated in future version, I can manage to develop it.