More global? I might be misunderstanding you but email is everywhere. It's one of the things that is recognisable by your parents, maybe even grandparents. Wouldn't it be simpler, instead of trying to build out @handle usage, to add more control to emails? Everyone has one, they're much more open since only twitter (I think) has @handle things, and much much much more accessible (and global). Gmail has fantastic filters already, a bit of a pain to set up but they work. Wouldn't an improvement on those be better?