Google AdSense program policies do not allow publishers to place Google ads in a pop-up, pop-under, or in an email but this "No AdSense in Email" rule may be relaxed for Premium AdSense Publishers.
Rajesh, who is member of a Jobs related mailing list on Yahoo groups, received an email containing Google Ads from some headhunter company that also happens to be a premium AdSense publisher.
The Google Ads in the email are extremely content-targeted just like AdSense for webpages. [The email message was related to vacancies in an automobile company and ads are about Ford Motors, General Motors and Hyundai]
Here's a screen capture from that email:
The only difference is that instead of linking to Advertisers website, these AdSense ads take you to the publisher's AdSense for search page that is prepopulated with Google Ads related to the keywords mentioned in the email.
Not sure how Google delivers contextual advertising in email messages - it's definitely not real time like AdSense because the search keywords are hard coded inside the email messages.
If Google opens up the program for a larger set of publishers, AdSense email ads may well be a wonderful opportunity for newsletters like Internet Tourbus and Windows Secrets to monetize using AdSense even if their websites don't qualify for Premium membership.