Mail Backup is a personal tool. Its only user is its owner, who uses it to keep a private copy of their own Gmail on their own server.
Mail Backup reads the owner's Gmail with read-only access (the gmail.readonly scope). It cannot send, change or delete email.
The email it reads is stored only on hardware the owner controls. It is not sold, shared or sent to anyone else, and it is not used for advertising.
The app is not offered to the public. No one else's data is collected.
Access can be revoked at any time at myaccount.google.com/permissions.