I can confirm, that adding
smtp[:authentication]
to Net::SMTP.start call and setting
DISCOURSE_SMTP_AUTHENTICATION: login
in app.yml fixes the issue for email test page.
I believe, that regular emails are send via mail library, and setting
DISCOURSE_SMTP_AUTHENTICATION: login
in app.yml is enough for the library to work correctly.
Still, I believe, that lib/tasks/emails.rake should be patched to use DISCOURSE_SMTP_AUTHENTICATION setting. This would save some unnecessary debugging .