Serving content-targeted ads in e-mail newsletters is a potential source of a large number of additional quality page-views for advertisers. (...) Regardless of the system used to serve ads with e-mail, such as e-mail newsletters, it may be desirable to (i) obtain e-mail content information so that useful, content-relevant, ads may be served, and (ii) provide ads in a format that can be rendered on and supported by a wide variety of e-mail clients/readers, or at least prevalent e-mail clients/readers. This may be challenging since many, if not most, of the more popular Web-based e-mail clients strip out IFRAMEs and JavaScript. (...)
The present invention may be used to serve content-targeted ads with e-mail messages, such as HTML e-mail messages, and may do so without needing to use IFRAMEs or JavaScript. The present invention may do so by (i) having the document publisher include a unique content identifier in the content, (ii) having a client device pass the unique content identifier to a content-relevant ad server in a content-relevant ad request, and (iii) having the content-relevant ad server use the unique contend identifier to identify previously registered content for purposes of determining content-relevant ads.
So their system relies on images or dynamically-generated image maps that serve ads related to the content of the newsletter. But many mail clients and webmail apps also block embedded images (unless you explicitly allow them).
A post from Search Engine Roundtable mentioned last year that "some Google AdSense publishers are allowed to place ads within email newsletters or other emails, with permission from Google". An advertiser confirms: "We have had AdSense ads in our newsletters for over a year now. These ads come in various formats and are essentially served by an image server".