This app uses the iOS filtering extension to mark messages as junk if they match certain criteria. It does NOT contact a server to process messages; all processing is done locally.
Filters are synced via iCloud. If you want to compile the app, you'll need to setup your developer account for the app and the app extension as well as create an iCloud container.
This repository is mostly here to show that the app published in the app store never sends your data anywhere to be processed.