Mailbox.org has a feature like that, where all incoming messages are PGP encrypted to your public key when they're received so there's only a small window where

Mailbox.org has a feature like that, where all incoming messages are PGP encrypted to your public key when they're received so there's only a small window where they have the plaintext (and the plaintext isn't stored long-term nor is it stored in a way where they can decrypt it). The problem is that fundamentally email still provides them the plaintext -- so they wouldn't need to add a new capability or break the design of their at-rest crypto, they would simply have to stash away emails that were requested by a warrant. The problem is that e2e messaging applications, where there is no way for the server to do this, are going to be in quite serious trouble. They would need to re-design their crypto so they can add additional "backdoor" device keys that let them access the messages, or provide backdoored binaries to the relevant target. And that's what TCNs will be used for -- to compel those kinds of backdoors.