Working on the client sounds good too! Your anti-spam idea would work for one use case of social media, friends and family keeping in touch, but as soon as some of the participants don't know each other that is not enough. Even using regular email you can't primarily rely on email providers to manage spam since they will block the list if there is too much spam, although initial moderation of new list members should often be sufficient. Encrypting the email to the recipient might or might not help with deliverability (but would be a good idea for privacy), but if it does that would make your system more attractive to spammers.

Reliably seeing all messages that your friends and family (or anyone you follow) post would be a big advantage vs many current social media sites but regular email providers would defeat that by randomly sending messages to spam (which you could fix if you provide the client) or rejecting or disappearing them (which you can't).

I think an email provider that aimed at the "keeping in touch with friends and famnily" side of social media might be able to do small-scale well. I think a big part of the issue with email is that most of the end user providers seem to view email as something they must provide but not something they care about, only the marketing/transaction email providers actually care about it with few exceptions (and those exceptions seem to still be stuck in the past; it is depressingly difficult to find a provider with a reasonable set of features even based on what could easily be provided today).