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. 

 Gmail supports any IMAP client and I think most other major providers do, too. 

 I think SMTP submission is a bit more strict at least in my experience with AWS SES 

 The “ability to move to a different client” should be marketed as “own your own data” by everybody in the space trying to monetize a client.

Then the whole ecosystem gains.  Rising tide lifts all boats.