Consider the way BitTorrent works. You don't have to know what your peers are going to be, or where. You don't have to trust your peers: as long as they serve blocks with correct hashes, you are safe to take data from them. Equally, they don't have to know you or trust you.
It, of course, does not work for email. OTOH protocols like briar [1] theoretically could.
[1]: https://en.wikipedia.org/wiki/Briar_(software)