There's a very logical reason on why the US government won't, and cannot tax e-mail.
1) Logically, the riots that would instill from thousands of online users would cost them more money than the tax would make.
2) No method of policing, e-mail cannot be controlled; meaning, I can send it from my public library's computer, church, work, home. the Internet Privacy Act prevents e-mail providers from releasing information such as how many e-mails where sent.
3) Not sure on the legal aspect of this, but the internet is a global domain, meaning the US Gov't has no jurisdiction on it. This is why bootlegging sites in Russia exist. It is a worldwide property and thus cannot be controlled by the Feds. Yes, one could argue that the e-mail servers are physically located on US soil and thus would fall under jurisdiction, BUT there are servers all over the world. People would just stop using any server based on the US and thus local companies would lose income.
4) No matter the dumb crap we've done, including allowing Hildog to run for Pres, our government is not stupid enough to even think they could pull this off.