1What is Newsletter Sender written in?
The server side is TypeScript, which is JavaScript with types. It runs as a Cloudflare Worker, and the data lives in Cloudflare's D1 database, which is SQLite.
The app screens are a single page of plain HTML, CSS and JavaScript, with no framework. That keeps it small and quick, and there's nothing for you to run on a server.
2What would happen if Cloudflare closed my account?
Your emails go out through your sending service, such as Amazon SES, so your sending and your reputation aren't affected.
Your newsletter's data lives in Cloudflare, but Newsletter Sender makes a backup every night. If you send with Amazon SES, it also keeps a copy in your own Amazon account, and you can download a backup any time.
To come back, install Newsletter Sender on a new Cloudflare account, go to Settings, Backups, Restore from file, and paste your sending key again. See Backups and recovery.
3Where are my backups kept?
In your own Cloudflare storage, with the last 30 kept. If you send with Amazon SES, a copy of each also goes to a private bucket in your own Amazon account for 31 days. Nobody else, including us, can see either. See Backups and recovery.