silverstripe-framework/_config/mailer.yml

---
Name: mailer
---
SilverStripe\Core\Injector\Injector:
  Symfony\Component\Mailer\MailerInterface:
    class: Symfony\Component\Mailer\Mailer
    constructor:
      transport: '%$Symfony\Component\Mailer\Transport\TransportInterface'
  Symfony\Component\EventDispatcher\EventDispatcherInterface.mailer:
    class: Symfony\Component\EventDispatcher\EventDispatcher
    calls:
      - [addSubscriber, ['%$SilverStripe\Control\Email\MailerSubscriber']]
  Symfony\Component\Mailer\Transport\TransportInterface:
    factory: Symfony\Component\Mailer\Transport
    factory_method: fromDsn
    constructor:
      dsn: 'sendmail://default'
      dispatcher: '%$Symfony\Component\EventDispatcher\EventDispatcherInterface.mailer'
---
Name: mailer-dsn-env
After: '*'
Only:
  envvarset: MAILER_DSN
---
SilverStripe\Core\Injector\Injector:
  Symfony\Component\Mailer\Transport\TransportInterface:
    constructor:
      dsn: '`MAILER_DSN`'