Account ids are created with the username and the second level domain under which the
app is installed e.g. `@$CLOUDRON-USERNAME@$CLOUDRON-APP-DOMAIN`.

For federation to work, the second level domain has to be configured to serve up the
`.well-known/domain.com/matrix` URI. See the 
[federation docs](https://cloudron.io/documentation/apps/synapse/) on
how to do this.