Now that you understand how this email app works, feel free to fork it and build your own email app!
Here are some potential features that you might want to build.
In addition to storing your saved emails in your root structured data, you could also store your outbox (the emails you send).
Currently you have to compose a new email if you want to reply to someone. You could add the ability to send replies.
You could also add the ability to forward an email to other users.
You could add the ability to upload file attachments using the Immutable Data API.
You could add the ability to send an email to multiple recipients. The email would need to be added to the appendable data of each recipient.
filter field of the appendable data, you could let the user specify a blacklist of a whitelist.
See the Filter API for more info.
Try to build an email app with a pre-built template such as this one.