silverstripe-webpack/app/_config/email.yml

---
Name: app-emailconfig
After:
  - '#emailconfig'
---
SilverStripe\Core\Injector\Injector:
  Swift_Transport:
    class: Swift_SendmailTransport

#SilverStripe\Control\Email\Email:
#  admin_email:
#    support@example.com: 'Support team'

#SilverStripe\Core\Injector\Injector:
#  Swift_Transport:
#    class: Swift_SmtpTransport
#    properties:
#      Host: smtp.host.com
#      Port: <port>
#      Encryption: tls
#    calls:
#      Username: [ setUsername, ['`APP_SMTP_USERNAME`'] ]
#      Password: [ setPassword, ['`APP_SMTP_PASSWORD`'] ]
#      AuthMode: [ setAuthMode, ['login'] ]