There is none
https://blog.thunderbird.net/2025/04/thundermail-and-thunder...
> Assist is an experiment, developed in partnership with Flower AI, a flexible open-source framework for scalable, privacy-preserving federated learning, that will enable users to take advantage of AI features. The hope is that processing can be done on devices that can support the models, and for devices that are not powerful enough to run the language models locally, we are making use of Flower Confidential Remote Compute in order to ensure private remote processing (very similar to Apple’s Private Cloud Compute).
> Given some users’ sensitivity to this, these types of features will always be optional and something that users will have to opt into. As a reminder, Thunderbird will never train AI with your data. The repo for Assist is not public yet, but it will be soon.