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".