According to code
SMTP should be configured in config/discourse.conf.
For me, I have these lines in it:
smtp_user_name = '...com'
smtp_address = '...com'
smtp_port = '587'
smtp_password = '...'
smtp_domain = '...com'
notification_email = 'noreply@....com'
Have you checked logs? For this custom install, the log is in production.log, production.log, puma.stdout.log under log dir.