The answer to spam is to charge a nominal fee for each e-mail, whether in digital currency (Bitcoin, Ripple) or proof-of-work (SHA, Bcrypt, etc.).
Bitmessage integrates the latter (proof-of-work) and is working nicely at this stage, although I believe it may have some structural issues that will make scaling it up to many thousands of nodes difficult.
Bitmessage integrates the latter (proof-of-work) and is working nicely at this stage, although I believe it may have some structural issues that will make scaling it up to many thousands of nodes difficult.