Email has some social scaling problems, but the technical scaling side is almost perfect - you push to the destination. Adding more destinations doesn't increase cost, and it splits the load.
Contrast this with e.g. Secure Scuttlebutt (under a normal setup). Adding more people to the connected graph (implying also a more densely connected graph) means exponentially more traffic and more storage for each member, due to the friend-of-a-friend proxying built in. It doesn't take long at all to reach the point where a normal internet connection can't keep up, and you fall farther and farther behind.
Mastodon falls somewhere in between - adding servers mostly splits load when you add instances, but caching behavior grows linearly and tends to happen simultaneously ~everywhere, and it doesn't take long before it's beyond what hobbyists can handle. It even affects outsiders, due to preloading link previews, which is a core privacy decision that's arguably part of the protocol.
Where does nostr fit in? Because "a protocol" describes ^ all of those. It's a mostly meaningless descriptor, beyond "probably not tied to a single company" (but not more than "probably").
