I was chatting with one of the blog authors here, not sure if I should mention which one, about how to add AdSense ads to emails. I am not sure if it is in accordance with the Google AdSense Terms and Conditions. So please read that and obtain approval from Google before trying this out. This is just for educational purposes.
In theory, you can do some form of AdSense cloaking, whereby you serve content from an HTML page and dynamically include the ads generated based on the content of that HTML page within the emails.
So if you have HTML emails on computer manufacturing, you might want to create static pages with the exact copy of the email. Then serve up those Google AdSense ads as if they were from the Web but in the email.
Very simple to do, but again, not sure if its in accordance with the AdSense terms. I personally won't be giving this one a shot. If you want to chat with others on the legalities of serving AdSense ads in your email, please visit DigitalPoints Forum - AdSense In Email.
