So what does a message actually cost in terms of dollars, power, time? How does it prevent low volume spam?

And that may prevent botspam, although I'm skeptical. But I seriously doubt it will have an effect on abuse (people stalking others, posting harrassing Tweets, etc.)

This. The project homepage literally claims (as a good thing) that "...no one can censor you. No one can remove your posts. Your account cannot be blocked."

So not only does this not help with abusive behavior, it states that abuse will live forever and be impossible to block or filter.

/me shudders

Almost like the Internet. So spooky

Rather different, actually. If I decide your inbound SMTP mail is spam, I can block it. Ditto for blog comments, XMPP contact requests, or really most any protocol in use on the public Internet b/c of course there will be griefers and spammers and all kinds of bad out there and we need tools to filter and protect against it.

You have the right to say anything you want (though not without consequences). Conversely, I should have the right to literally not see/hear it once I've decided it is causing me harm.

Claiming that your platform/protocol makes this impossible just sounds naive at best and nefarious at worst to me.