This is pretty easy to setup, in your app.yml:
## TODO: The mailserver this Discourse instance will use
DISCOURSE_SMTP_ADDRESS: email-smtp.us-east-1.amazonaws.com
DISCOURSE_SMTP_PORT: 587
DISCOURSE_SMTP_USER_NAME: CREDIENTIALS
DISCOURSE_SMTP_PASSWORD: CREDIENTIALSPW
DISCOURSE_SMTP_ENABLE_START_TLS: true
Worked straight out of the box for me.
Edit:
To apply these changes you will need to run: ./launcher rebuild app