as posted, it's currently not allowed.. But on ShoeMoney's podcast a few weeks ago he interviewed Brian Axe from the Google Adsense team and he hinted that Adsense for Email might be a posiblity in the near future if they could work out a few issues - such as how to deal with the obvious potential spam problems that would arise..
That's actually a great tip, thanks! You've just convinced me that I need to start tuning in to those webcasts
I think it is not allowed for a regular publisher Adsense would have a deal going with big publishers, probably meaning the emails have to reach a certain quality of content in order to be allowed to add adsense - content which Google would know is good from certain sites (ones that NEVER send bad emails etc, only useful ones with articles etc)