Outline only uses SSO for authentication. The solution when self hosting is use a private keycloak server [1]. This allows you to do email based auth.

[1] https://www.keycloak.org/

You can self host SSO. You maybe do already. Nextcloud, Gitea and Mattermost offer these services. We tried to use Mattermost but it didn't work. We went with Gitea. I only discovered later than Nextcloud also did.

I really would like to try this out locally.

Then I realized it seems like it can’t without an internet connection.