Here is what my config looks like:
## TODO: The SMTP mail server used to validate new accounts and send notifications
# SMTP ADDRESS, username, and password are required
# WARNING the char '#' in SMTP password can cause problems!
DISCOURSE_SMTP_ADDRESS: smtpout.secureserver.net
DISCOURSE_SMTP_PORT: 587
DISCOURSE_SMTP_USER_NAME: xxxxxxxxx@cmmchub.com
DISCOURSE_SMTP_PASSWORD: "Password"
#DISCOURSE_SMTP_ENABLE_START_TLS: true
#DISCOURSE_SMTP_OPENSSL_VERIFY_MODE: none
DISCOURSE_SMTP_ENABLE_START_TLS: true # (optional, default true)