> SMTP will delete spam

Can you point to the part on the SMTP RFC that specifies the spam deletion behaviour?

> XMPP servers will kick out spammers and harassers

Nothing prevents nostr servers from kicking out based on account or IP address or message content

> The problem with Twitter is that unlike RSS, it shows content from people you did not subscribe to.

This is not Twitter, any client can decide which content to show. And there are clients that do not show any content from people you don't explicitly follow.