Private messages are end-to-end encrypted, always.
You have to follow somebody to get messages from them, so you won't get spammed.
Your mail can be public broadcasts or private, and you'll only see replies by people you follow.
The datastructure is a global mesh of append-only logs, which can support new types of data (not just "mail").
Users are not bound to one server/host (what we call "pubs") and do not have to trust the servers.
It's very easy to setup and maintain your own pub. Private messages are end-to-end encrypted, always.
What is the minimum we can get away with to create an ethereum/ssb integration, that gets us somewhere. It doesn't need to be perfect yet, or have all the features yet, it just has to be something that does something, that is a baby step towards the big idea.