Tim:
Blacklisting (which is what my suggestion was) has its problems. Though a clear use of com for commercial, would have made it very easy to avoid all commercial mail, if you wanted to, and if it was possible to force commercial users to use it. The last "if" really being the nail in the coffin for it.
Bruno Wolff III:
It wouldn't work as well as you might think. Most of teh junk comes from spammers who would have no problem sending email that looked like it came from individuals if they thought it would help people see their email.
I know, I said that already. Read that paragraph, again.