I'm not sure if it's even possible to build a twitter-like system with distributed servers in the vein of e-mail. It's possible for e-mail because it's a one-way system. Twitter has the concept of following a user, and that user's tweets appear to me. It's a 2-way system that needs to constantly be kept in sync. Even if it's possible to do, it won't be easy, with loads of extra technical challenges that SMTP doesn't have.