It’s probably set in your container configuration file, the default contents in the template are:
DISCOURSE_SMTP_ADDRESS: smtp.example.com
#DISCOURSE_SMTP_PORT: 587
DISCOURSE_SMTP_USER_NAME: user@example.com
DISCOURSE_SMTP_PASSWORD: pa$$word
#DISCOURSE_SMTP_ENABLE_START_TLS: true # (optional, default true)
#DISCOURSE_SMTP_DOMAIN: discourse.example.com # (required by some providers)
#DISCOURSE_NOTIFICATION_EMAIL: noreply@discourse.example.com # (address to send notifications from)