[lijogdfljk]

You can, though, kinda.

In your mailman example, it would be like the FBI announcing exactly what the envelope looks like, uniquely identifiable, and if you encounter this letter to report it immediately or else your complicit. Or at least, that's the fear.

Yes, if it's encrypted you have no idea, but if anyone ever found out what was in it you can know who ever transmitted it. Unless everyone re-encrypts it repeatedly.. which then makes content address-ability awkward.