I have created a new email protocol for user-triggered email forwarding that proves email authenticity. Forwarded between different accounts of different users.

It is heavily influenced by the ARC protocol for automatic forwarding of emails between different accounts of the same user.

The protocol has options to prove the authenticity of an entire chain of forwarded emails between different users and also manage user consent.

I linked the docs here, with the protocol description. You can also find a technical demo video there.

I will announce when the Golang implementation is ready to be open-sourced: upvote if you are interested in this.