Nostr’s “super power” is the ability to move to a different client.  Success leads to centralization and capture; a great client or relay (or email app) is going to attract more users. In the case of SMTP, if I’m understanding your point, users are mostly locked into a specific set of providers because those same providers filter out “bad actors”. If that sort of dynamic appears in nostr, user have an opportunity to thwart that by changing clients and/or relays. Will they? Uncertain, but the ability exists in nostr unlike other protocols and platforms.