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A Global, Open, Cryptographically-Signed Bulletin Board

Email doesn't have this inherent cryptographic identity baked in.

emails with a dozen of servers where each one stores everything, if it wants

btw. smtp is just for sending emails. Then you also need imap for receiving those.

Well, no and perhaps yes:

when use it as an analogy, make sure to point out that with email, your address is tied to a server/provider whereas with Nostr the whole point is that that is not the case.