There's something crappy-ish and similar for email with DKIM. That doesn't sound the same, because it's the server signing, not the sender -- but in this protoc

There's something crappy-ish and similar for email with DKIM. That doesn't sound the same, because it's the server signing, not the sender -- but in this protocol you're kinda already trusting the server anyway. The critical difference is that DKIM is not protected with WebPKI.