> How exactly does that work with end-to-end encryption?
One way would be for Apple to offer a web API endpoint for companies that have signed an agreement with them (covering rate limits, server identification certificates, liability, etc.).
In fact, the API could be implemented in iOS itself, so you could have something like "Signal support for iMessage" as an app in the App Store, which would basically be a headless version of the Signal app which delegates all the UI tasks (mainly message display and input) to the iMessage app.
One way would be for Apple to offer a web API endpoint for companies that have signed an agreement with them (covering rate limits, server identification certificates, liability, etc.).
In fact, the API could be implemented in iOS itself, so you could have something like "Signal support for iMessage" as an app in the App Store, which would basically be a headless version of the Signal app which delegates all the UI tasks (mainly message display and input) to the iMessage app.