mastodon-app/POSTINSTALL.md

Accounts are created with the username and the subdomain under which this app is installed e.g. @$CLOUDRON-USERNAME@$CLOUDRON-APP-FQDN. Mastodon does not allow changing the domain part of the account later. See the docs for more information, if you want to change this domain.

**NOTE:** * Mastodon has [restrictions](https://docs.cloudron.io/apps/mastodon/#username-restriction) on usernames that might prevent some users from logging in.

• External registration does not work well when Cloudron user management is enabled.