You’ll want to check the /logs URL.
If you’re debugging, you might be farther ahead trying the rake 'emails:test[your_email]' task.
You can also set environment variables to try different things without needing a container rebuild, e.g.:
$ DISCOURSE_SMTP_PORT=587 DISCOURSE_SMTP_USER_NAME=bilbo DISCOURSE_SMTP_PASSWORD=ring rake emails:test'[frodo@shire.net]'