Name a crypto-related technical concept that can't be explained using an analogy to mail.  I'll wait.

Admittedly a bit hard to think about. Some things translate down to signaling priority, other things involve messaging to other jurisdictions, but ultimately it comes down to what consensus is on everything that has occurred which in the conventional mail system, is inconceivable since there are unbreakable trust assumptions in the global env of the system (well besides some recovery effort if the organization collapses and requires government/external intervention)