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-DOMAIN. Note that Mastodon does NOT allow this subdomain to be changed later as it breaks federation. So, you have to install Mastodon in the correct subdomain from the get-go.

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