A tax on email would never, ever work. People would simply abandon it for “cached” instant messaging, STW (secure transient webpages), or other technologies that allow people to communicate without invoking SMTP and the “email” moniker. Nothing short of a per-kilobit tax would stop people from working around the tax, and a per-kilobit tax would be viciously opposed by everyone from the telephone companies (who want to deliver TV over broadband) to the cable companies (who want to deliver telephone service over broadband) to every single web owner and online media service. We would revert back to 1995, when webpages were simple and images were optional because every KB was precious (back then because connections were slow, now because they’re expensive).
That won't work either, once internet is delivered over power lines.