You mean *unlike* SMTP? Signatures and relay hints

SMTP works at scale but is not sufficiently decentralized. nostr doesnt need all the header stuff just dynamic relay reputation

Easy, same way the experimental feature in element / adblocks do it. You create block lists people can subscribe to in their client regardless of which relay. Both blacklist and whitelist modes. That way people can tailor the experience tp their liking using lists they trust. If a list gets overly censorious you just replace it.